mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()

Patch series "mm: minor cleanups around NUMA hinting".

Working on some GUP cleanups (e.g., getting rid of some FOLL_ flags) and
preparing for other GUP changes (getting rid of FOLL_FORCE|FOLL_WRITE for
for taking a R/O longterm pin), this is something I can easily send out
independently.

Get rid of FOLL_NUMA, allow FOLL_FORCE access to PROT_NONE mapped pages in
GUP-fast, and fixup some documentation around NUMA hinting.


This patch (of 3):

No need for a special flag that is not even properly documented to be
internal-only.

Let's just factor this check out and get rid of this flag.  The separate
function has the nice benefit that we can centralize comments.

Link: https://lkml.kernel.org/r/20220825164659.89824-2-david@redhat.com
Link: https://lkml.kernel.org/r/20220825164659.89824-1-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -2933,7 +2933,6 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 				 * and return without waiting upon it */
 #define FOLL_NOFAULT	0x80	/* do not fault in pages */
 #define FOLL_HWPOISON	0x100	/* check page is hwpoisoned */
-#define FOLL_NUMA	0x200	/* force NUMA hinting page fault */
 #define FOLL_MIGRATION	0x400	/* wait for page to replace migration entry */
 #define FOLL_TRIED	0x800	/* a retry, previous pass started an IO */
 #define FOLL_REMOTE	0x2000	/* we are working on non-current tsk/mm */
@@ -3054,6 +3053,21 @@ static inline bool gup_must_unshare(unsigned int flags, struct page *page)
 	return !PageAnonExclusive(page);
 }
 
+/*
+ * Indicates whether GUP can follow a PROT_NONE mapped page, or whether
+ * a (NUMA hinting) fault is required.
+ */
+static inline bool gup_can_follow_protnone(unsigned int flags)
+{
+	/*
+	 * FOLL_FORCE has to be able to make progress even if the VMA is
+	 * inaccessible. Further, FOLL_FORCE access usually does not represent
+	 * application behaviour and we should avoid triggering NUMA hinting
+	 * faults.
+	 */
+	return flags & FOLL_FORCE;
+}
+
 typedef int (*pte_fn_t)(pte_t *pte, unsigned long addr, void *data);
 extern int apply_to_page_range(struct mm_struct *mm, unsigned long address,
 			       unsigned long size, pte_fn_t fn, void *data);

mm/gup.c

@@ -561,7 +561,7 @@ retry:
 		migration_entry_wait(mm, pmd, address);
 		goto retry;
 	}
-	if ((flags & FOLL_NUMA) && pte_protnone(pte))
+	if (pte_protnone(pte) && !gup_can_follow_protnone(flags))
 		goto no_page;
 
 	page = vm_normal_page(vma, address, pte);
@@ -714,7 +714,7 @@ retry:
 	if (likely(!pmd_trans_huge(pmdval)))
 		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
 
-	if ((flags & FOLL_NUMA) && pmd_protnone(pmdval))
+	if (pmd_protnone(pmdval) && !gup_can_follow_protnone(flags))
 		return no_page_table(vma, flags);
 
 retry_locked:
@@ -1160,14 +1160,6 @@ static long __get_user_pages(struct mm_struct *mm,
 
 	VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));
 
-	/*
-	 * If FOLL_FORCE is set then do not force a full fault as the hinting
-	 * fault information is unrelated to the reference behaviour of a task
-	 * using the address space
-	 */
-	if (!(gup_flags & FOLL_FORCE))
-		gup_flags |= FOLL_NUMA;
-
 	do {
 		struct page *page;
 		unsigned int foll_flags = gup_flags;

mm/huge_memory.c

@@ -1447,7 +1447,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 		return ERR_PTR(-EFAULT);
 
 	/* Full NUMA hinting faults to serialise migration in fault paths */
-	if ((flags & FOLL_NUMA) && pmd_protnone(*pmd))
+	if (pmd_protnone(*pmd) && !gup_can_follow_protnone(flags))
 		return NULL;
 
 	if (!pmd_write(*pmd) && gup_must_unshare(flags, page))