mm: speculative page references
If we can be sure that elevating the page_count on a pagecache page will pin it, we can speculatively run this operation, and subsequently check to see if we hit the right page rather than relying on holding a lock or otherwise pinning a reference to the page. This can be done if get_page/put_page behaves consistently throughout the whole tree (ie. if we "get" the page after it has been used for something else, we must be able to free it with a put_page). Actually, there is a period where the count behaves differently: when the page is free or if it is a constituent page of a compound page. We need an atomic_inc_not_zero operation to ensure we don't try to grab the page in either case. This patch introduces the core locking protocol to the pagecache (ie. adds page_cache_get_speculative, and tweaks some update-side code to make it work). Thanks to Hugh for pointing out an improvement to the algorithm setting page_count to zero when we have control of all references, in order to hold off speculative getters. [kamezawa.hiroyu@jp.fujitsu.com: fix migration_entry_wait()] [hugh@veritas.com: fix add_to_page_cache] [akpm@linux-foundation.org: repair a comment] Signed-off-by: Nick Piggin <npiggin@suse.de> Cc: Jeff Garzik <jeff@garzik.org> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Paul Mackerras <paulus@samba.org> Cc: Hugh Dickins <hugh@veritas.com> Cc: "Paul E. McKenney" <paulmck@us.ibm.com> Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Signed-off-by: Hugh Dickins <hugh@veritas.com> Acked-by: Nick Piggin <npiggin@suse.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Nick Piggin
Linus Torvalds
Родитель
47feff2c8e
Коммит
e286781d5f
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@ -576,6 +576,18 @@ static void cas_spare_recover(struct cas *cp, const gfp_t flags)
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list_for_each_safe(elem, tmp, &list) {
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cas_page_t *page = list_entry(elem, cas_page_t, list);
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/*
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* With the lockless pagecache, cassini buffering scheme gets
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* slightly less accurate: we might find that a page has an
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* elevated reference count here, due to a speculative ref,
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* and skip it as in-use. Ideally we would be able to reclaim
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* it. However this would be such a rare case, it doesn't
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* matter too much as we should pick it up the next time round.
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*
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* Importantly, if we find that the page has a refcount of 1
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* here (our refcount), then we know it is definitely not inuse
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* so we can reuse it.
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*/
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if (page_count(page->buffer) > 1)
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continue;
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@ -12,6 +12,7 @@
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#include <asm/uaccess.h>
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#include <linux/gfp.h>
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#include <linux/bitops.h>
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#include <linux/hardirq.h> /* for in_interrupt() */
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/*
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* Bits in mapping->flags. The lower __GFP_BITS_SHIFT bits are the page
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@ -62,6 +63,98 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
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#define page_cache_release(page) put_page(page)
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void release_pages(struct page **pages, int nr, int cold);
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/*
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* speculatively take a reference to a page.
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* If the page is free (_count == 0), then _count is untouched, and 0
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* is returned. Otherwise, _count is incremented by 1 and 1 is returned.
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*
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* This function must be called inside the same rcu_read_lock() section as has
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* been used to lookup the page in the pagecache radix-tree (or page table):
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* this allows allocators to use a synchronize_rcu() to stabilize _count.
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*
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* Unless an RCU grace period has passed, the count of all pages coming out
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* of the allocator must be considered unstable. page_count may return higher
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* than expected, and put_page must be able to do the right thing when the
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* page has been finished with, no matter what it is subsequently allocated
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* for (because put_page is what is used here to drop an invalid speculative
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* reference).
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*
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* This is the interesting part of the lockless pagecache (and lockless
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* get_user_pages) locking protocol, where the lookup-side (eg. find_get_page)
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* has the following pattern:
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* 1. find page in radix tree
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* 2. conditionally increment refcount
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* 3. check the page is still in pagecache (if no, goto 1)
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*
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* Remove-side that cares about stability of _count (eg. reclaim) has the
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* following (with tree_lock held for write):
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* A. atomically check refcount is correct and set it to 0 (atomic_cmpxchg)
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* B. remove page from pagecache
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* C. free the page
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*
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* There are 2 critical interleavings that matter:
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* - 2 runs before A: in this case, A sees elevated refcount and bails out
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* - A runs before 2: in this case, 2 sees zero refcount and retries;
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* subsequently, B will complete and 1 will find no page, causing the
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* lookup to return NULL.
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*
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* It is possible that between 1 and 2, the page is removed then the exact same
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* page is inserted into the same position in pagecache. That's OK: the
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* old find_get_page using tree_lock could equally have run before or after
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* such a re-insertion, depending on order that locks are granted.
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*
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* Lookups racing against pagecache insertion isn't a big problem: either 1
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* will find the page or it will not. Likewise, the old find_get_page could run
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* either before the insertion or afterwards, depending on timing.
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*/
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static inline int page_cache_get_speculative(struct page *page)
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{
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VM_BUG_ON(in_interrupt());
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#if !defined(CONFIG_SMP) && defined(CONFIG_CLASSIC_RCU)
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# ifdef CONFIG_PREEMPT
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VM_BUG_ON(!in_atomic());
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# endif
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/*
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* Preempt must be disabled here - we rely on rcu_read_lock doing
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* this for us.
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*
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* Pagecache won't be truncated from interrupt context, so if we have
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* found a page in the radix tree here, we have pinned its refcount by
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* disabling preempt, and hence no need for the "speculative get" that
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* SMP requires.
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*/
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VM_BUG_ON(page_count(page) == 0);
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atomic_inc(&page->_count);
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#else
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if (unlikely(!get_page_unless_zero(page))) {
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/*
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* Either the page has been freed, or will be freed.
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* In either case, retry here and the caller should
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* do the right thing (see comments above).
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*/
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return 0;
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}
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#endif
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VM_BUG_ON(PageTail(page));
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return 1;
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}
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static inline int page_freeze_refs(struct page *page, int count)
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{
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return likely(atomic_cmpxchg(&page->_count, count, 0) == count);
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}
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static inline void page_unfreeze_refs(struct page *page, int count)
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{
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VM_BUG_ON(page_count(page) != 0);
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VM_BUG_ON(count == 0);
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atomic_set(&page->_count, count);
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}
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#ifdef CONFIG_NUMA
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extern struct page *__page_cache_alloc(gfp_t gfp);
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#else
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@ -133,13 +226,29 @@ static inline struct page *read_mapping_page(struct address_space *mapping,
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return read_cache_page(mapping, index, filler, data);
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}
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int add_to_page_cache(struct page *page, struct address_space *mapping,
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int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
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pgoff_t index, gfp_t gfp_mask);
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int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
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pgoff_t index, gfp_t gfp_mask);
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extern void remove_from_page_cache(struct page *page);
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extern void __remove_from_page_cache(struct page *page);
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/*
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* Like add_to_page_cache_locked, but used to add newly allocated pages:
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* the page is new, so we can just run SetPageLocked() against it.
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*/
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static inline int add_to_page_cache(struct page *page,
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struct address_space *mapping, pgoff_t offset, gfp_t gfp_mask)
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{
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int error;
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SetPageLocked(page);
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error = add_to_page_cache_locked(page, mapping, offset, gfp_mask);
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if (unlikely(error))
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ClearPageLocked(page);
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return error;
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}
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/*
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* Return byte-offset into filesystem object for page.
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*/
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32
mm/filemap.c
32
mm/filemap.c
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@ -442,39 +442,43 @@ int filemap_write_and_wait_range(struct address_space *mapping,
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}
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/**
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* add_to_page_cache - add newly allocated pagecache pages
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* add_to_page_cache_locked - add a locked page to the pagecache
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* @page: page to add
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* @mapping: the page's address_space
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* @offset: page index
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* @gfp_mask: page allocation mode
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*
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* This function is used to add newly allocated pagecache pages;
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* the page is new, so we can just run SetPageLocked() against it.
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* The other page state flags were set by rmqueue().
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*
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* This function is used to add a page to the pagecache. It must be locked.
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* This function does not add the page to the LRU. The caller must do that.
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*/
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int add_to_page_cache(struct page *page, struct address_space *mapping,
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int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
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pgoff_t offset, gfp_t gfp_mask)
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{
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int error = mem_cgroup_cache_charge(page, current->mm,
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int error;
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VM_BUG_ON(!PageLocked(page));
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error = mem_cgroup_cache_charge(page, current->mm,
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gfp_mask & ~__GFP_HIGHMEM);
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if (error)
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goto out;
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error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
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if (error == 0) {
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page_cache_get(page);
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page->mapping = mapping;
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page->index = offset;
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write_lock_irq(&mapping->tree_lock);
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error = radix_tree_insert(&mapping->page_tree, offset, page);
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if (!error) {
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page_cache_get(page);
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SetPageLocked(page);
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page->mapping = mapping;
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page->index = offset;
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if (likely(!error)) {
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mapping->nrpages++;
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__inc_zone_page_state(page, NR_FILE_PAGES);
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} else
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} else {
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page->mapping = NULL;
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mem_cgroup_uncharge_cache_page(page);
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page_cache_release(page);
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}
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write_unlock_irq(&mapping->tree_lock);
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radix_tree_preload_end();
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@ -483,7 +487,7 @@ int add_to_page_cache(struct page *page, struct address_space *mapping,
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out:
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return error;
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}
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EXPORT_SYMBOL(add_to_page_cache);
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EXPORT_SYMBOL(add_to_page_cache_locked);
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int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
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pgoff_t offset, gfp_t gfp_mask)
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20
mm/migrate.c
20
mm/migrate.c
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@ -285,7 +285,15 @@ void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
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page = migration_entry_to_page(entry);
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get_page(page);
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/*
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* Once radix-tree replacement of page migration started, page_count
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* *must* be zero. And, we don't want to call wait_on_page_locked()
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* against a page without get_page().
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* So, we use get_page_unless_zero(), here. Even failed, page fault
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* will occur again.
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*/
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if (!get_page_unless_zero(page))
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goto out;
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pte_unmap_unlock(ptep, ptl);
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wait_on_page_locked(page);
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put_page(page);
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@ -305,6 +313,7 @@ out:
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static int migrate_page_move_mapping(struct address_space *mapping,
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struct page *newpage, struct page *page)
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{
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int expected_count;
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void **pslot;
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if (!mapping) {
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@ -319,12 +328,18 @@ static int migrate_page_move_mapping(struct address_space *mapping,
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pslot = radix_tree_lookup_slot(&mapping->page_tree,
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page_index(page));
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if (page_count(page) != 2 + !!PagePrivate(page) ||
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expected_count = 2 + !!PagePrivate(page);
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if (page_count(page) != expected_count ||
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(struct page *)radix_tree_deref_slot(pslot) != page) {
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write_unlock_irq(&mapping->tree_lock);
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return -EAGAIN;
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}
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if (!page_freeze_refs(page, expected_count)) {
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write_unlock_irq(&mapping->tree_lock);
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return -EAGAIN;
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}
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/*
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* Now we know that no one else is looking at the page.
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*/
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@ -338,6 +353,7 @@ static int migrate_page_move_mapping(struct address_space *mapping,
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radix_tree_replace_slot(pslot, newpage);
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page_unfreeze_refs(page, expected_count);
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/*
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* Drop cache reference from old page.
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* We know this isn't the last reference.
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|
|
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@ -936,7 +936,7 @@ found:
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spin_lock(&info->lock);
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ptr = shmem_swp_entry(info, idx, NULL);
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if (ptr && ptr->val == entry.val) {
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error = add_to_page_cache(page, inode->i_mapping,
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error = add_to_page_cache_locked(page, inode->i_mapping,
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idx, GFP_NOWAIT);
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/* does mem_cgroup_uncharge_cache_page on error */
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} else /* we must compensate for our precharge above */
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@ -1301,8 +1301,8 @@ repeat:
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SetPageUptodate(filepage);
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set_page_dirty(filepage);
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swap_free(swap);
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} else if (!(error = add_to_page_cache(
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swappage, mapping, idx, GFP_NOWAIT))) {
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} else if (!(error = add_to_page_cache_locked(swappage, mapping,
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idx, GFP_NOWAIT))) {
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info->flags |= SHMEM_PAGEIN;
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shmem_swp_set(info, entry, 0);
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shmem_swp_unmap(entry);
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|
|
|
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|
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@ -64,7 +64,7 @@ void show_swap_cache_info(void)
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|||
}
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|
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/*
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||||
* add_to_swap_cache resembles add_to_page_cache on swapper_space,
|
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* add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,
|
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* but sets SwapCache flag and private instead of mapping and index.
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*/
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int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
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|
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@ -76,19 +76,26 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
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BUG_ON(PagePrivate(page));
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error = radix_tree_preload(gfp_mask);
|
||||
if (!error) {
|
||||
page_cache_get(page);
|
||||
SetPageSwapCache(page);
|
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set_page_private(page, entry.val);
|
||||
|
||||
write_lock_irq(&swapper_space.tree_lock);
|
||||
error = radix_tree_insert(&swapper_space.page_tree,
|
||||
entry.val, page);
|
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if (!error) {
|
||||
page_cache_get(page);
|
||||
SetPageSwapCache(page);
|
||||
set_page_private(page, entry.val);
|
||||
if (likely(!error)) {
|
||||
total_swapcache_pages++;
|
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__inc_zone_page_state(page, NR_FILE_PAGES);
|
||||
INC_CACHE_INFO(add_total);
|
||||
}
|
||||
write_unlock_irq(&swapper_space.tree_lock);
|
||||
radix_tree_preload_end();
|
||||
|
||||
if (unlikely(error)) {
|
||||
set_page_private(page, 0UL);
|
||||
ClearPageSwapCache(page);
|
||||
page_cache_release(page);
|
||||
}
|
||||
}
|
||||
return error;
|
||||
}
|
||||
|
|
|
|||
74
mm/vmscan.c
74
mm/vmscan.c
|
|
@ -391,12 +391,10 @@ static pageout_t pageout(struct page *page, struct address_space *mapping,
|
|||
}
|
||||
|
||||
/*
|
||||
* Attempt to detach a locked page from its ->mapping. If it is dirty or if
|
||||
* someone else has a ref on the page, abort and return 0. If it was
|
||||
* successfully detached, return 1. Assumes the caller has a single ref on
|
||||
* this page.
|
||||
* Same as remove_mapping, but if the page is removed from the mapping, it
|
||||
* gets returned with a refcount of 0.
|
||||
*/
|
||||
int remove_mapping(struct address_space *mapping, struct page *page)
|
||||
static int __remove_mapping(struct address_space *mapping, struct page *page)
|
||||
{
|
||||
BUG_ON(!PageLocked(page));
|
||||
BUG_ON(mapping != page_mapping(page));
|
||||
|
|
@ -427,24 +425,24 @@ int remove_mapping(struct address_space *mapping, struct page *page)
|
|||
* Note that if SetPageDirty is always performed via set_page_dirty,
|
||||
* and thus under tree_lock, then this ordering is not required.
|
||||
*/
|
||||
if (unlikely(page_count(page) != 2))
|
||||
if (!page_freeze_refs(page, 2))
|
||||
goto cannot_free;
|
||||
smp_rmb();
|
||||
if (unlikely(PageDirty(page)))
|
||||
/* note: atomic_cmpxchg in page_freeze_refs provides the smp_rmb */
|
||||
if (unlikely(PageDirty(page))) {
|
||||
page_unfreeze_refs(page, 2);
|
||||
goto cannot_free;
|
||||
}
|
||||
|
||||
if (PageSwapCache(page)) {
|
||||
swp_entry_t swap = { .val = page_private(page) };
|
||||
__delete_from_swap_cache(page);
|
||||
write_unlock_irq(&mapping->tree_lock);
|
||||
swap_free(swap);
|
||||
__put_page(page); /* The pagecache ref */
|
||||
return 1;
|
||||
} else {
|
||||
__remove_from_page_cache(page);
|
||||
write_unlock_irq(&mapping->tree_lock);
|
||||
}
|
||||
|
||||
__remove_from_page_cache(page);
|
||||
write_unlock_irq(&mapping->tree_lock);
|
||||
__put_page(page);
|
||||
return 1;
|
||||
|
||||
cannot_free:
|
||||
|
|
@ -452,6 +450,26 @@ cannot_free:
|
|||
return 0;
|
||||
}
|
||||
|
||||
/*
|
||||
* Attempt to detach a locked page from its ->mapping. If it is dirty or if
|
||||
* someone else has a ref on the page, abort and return 0. If it was
|
||||
* successfully detached, return 1. Assumes the caller has a single ref on
|
||||
* this page.
|
||||
*/
|
||||
int remove_mapping(struct address_space *mapping, struct page *page)
|
||||
{
|
||||
if (__remove_mapping(mapping, page)) {
|
||||
/*
|
||||
* Unfreezing the refcount with 1 rather than 2 effectively
|
||||
* drops the pagecache ref for us without requiring another
|
||||
* atomic operation.
|
||||
*/
|
||||
page_unfreeze_refs(page, 1);
|
||||
return 1;
|
||||
}
|
||||
return 0;
|
||||
}
|
||||
|
||||
/*
|
||||
* shrink_page_list() returns the number of reclaimed pages
|
||||
*/
|
||||
|
|
@ -598,18 +616,34 @@ static unsigned long shrink_page_list(struct list_head *page_list,
|
|||
if (PagePrivate(page)) {
|
||||
if (!try_to_release_page(page, sc->gfp_mask))
|
||||
goto activate_locked;
|
||||
if (!mapping && page_count(page) == 1)
|
||||
goto free_it;
|
||||
if (!mapping && page_count(page) == 1) {
|
||||
unlock_page(page);
|
||||
if (put_page_testzero(page))
|
||||
goto free_it;
|
||||
else {
|
||||
/*
|
||||
* rare race with speculative reference.
|
||||
* the speculative reference will free
|
||||
* this page shortly, so we may
|
||||
* increment nr_reclaimed here (and
|
||||
* leave it off the LRU).
|
||||
*/
|
||||
nr_reclaimed++;
|
||||
continue;
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
if (!mapping || !remove_mapping(mapping, page))
|
||||
if (!mapping || !__remove_mapping(mapping, page))
|
||||
goto keep_locked;
|
||||
|
||||
free_it:
|
||||
unlock_page(page);
|
||||
free_it:
|
||||
nr_reclaimed++;
|
||||
if (!pagevec_add(&freed_pvec, page))
|
||||
__pagevec_release_nonlru(&freed_pvec);
|
||||
if (!pagevec_add(&freed_pvec, page)) {
|
||||
__pagevec_free(&freed_pvec);
|
||||
pagevec_reinit(&freed_pvec);
|
||||
}
|
||||
continue;
|
||||
|
||||
activate_locked:
|
||||
|
|
@ -623,7 +657,7 @@ keep:
|
|||
}
|
||||
list_splice(&ret_pages, page_list);
|
||||
if (pagevec_count(&freed_pvec))
|
||||
__pagevec_release_nonlru(&freed_pvec);
|
||||
__pagevec_free(&freed_pvec);
|
||||
count_vm_events(PGACTIVATE, pgactivate);
|
||||
return nr_reclaimed;
|
||||
}
|
||||
|
|
|
|||
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