mm: Make compound_pincount always available

Move compound_pincount from the third page to the second page, which
means it's available for all compound pages.  That lets us delete
hpage_pincount_available().

On 32-bit systems, there isn't enough space for both compound_pincount
and compound_nr in the second page (it would collide with page->private,
which is in use for pages in the swap cache), so revert the optimisation
of storing both compound_order and compound_nr on 32-bit systems.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>

Documentation/core-api/pin_user_pages.rst

@@ -55,18 +55,18 @@ flags the caller provides. The caller is required to pass in a non-null struct
 pages* array, and the function then pins pages by incrementing each by a special
 value: GUP_PIN_COUNTING_BIAS.
 
-For huge pages (and in fact, any compound page of more than 2 pages), the
-GUP_PIN_COUNTING_BIAS scheme is not used. Instead, an exact form of pin counting
-is achieved, by using the 3rd struct page in the compound page. A new struct
-page field, hpage_pinned_refcount, has been added in order to support this.
+For compound pages, the GUP_PIN_COUNTING_BIAS scheme is not used. Instead,
+an exact form of pin counting is achieved, by using the 2nd struct page
+in the compound page. A new struct page field, compound_pincount, has
+been added in order to support this.
 
 This approach for compound pages avoids the counting upper limit problems that
 are discussed below. Those limitations would have been aggravated severely by
 huge pages, because each tail page adds a refcount to the head page. And in
-fact, testing revealed that, without a separate hpage_pinned_refcount field,
+fact, testing revealed that, without a separate compound_pincount field,
 page overflows were seen in some huge page stress tests.
 
-This also means that huge pages and compound pages (of order > 1) do not suffer
+This also means that huge pages and compound pages do not suffer
 from the false positives problem that is mentioned below.::
 
  Function
@@ -264,9 +264,9 @@ place.)
 Other diagnostics
 =================
 
-dump_page() has been enhanced slightly, to handle these new counting fields, and
-to better report on compound pages in general. Specifically, for compound pages
-with order > 1, the exact (hpage_pinned_refcount) pincount is reported.
+dump_page() has been enhanced slightly, to handle these new counting
+fields, and to better report on compound pages in general. Specifically,
+for compound pages, the exact (compound_pincount) pincount is reported.
 
 References
 ==========

include/linux/mm.h

@@ -887,17 +887,6 @@ static inline void destroy_compound_page(struct page *page)
 	compound_page_dtors[page[1].compound_dtor](page);
 }
 
-static inline bool hpage_pincount_available(struct page *page)
-{
-	/*
-	 * Can the page->hpage_pinned_refcount field be used? That field is in
-	 * the 3rd page of the compound page, so the smallest (2-page) compound
-	 * pages cannot support it.
-	 */
-	page = compound_head(page);
-	return PageCompound(page) && compound_order(page) > 1;
-}
-
 static inline int head_compound_pincount(struct page *head)
 {
 	return atomic_read(compound_pincount_ptr(head));
@@ -905,7 +894,7 @@ static inline int head_compound_pincount(struct page *head)
 
 static inline int compound_pincount(struct page *page)
 {
-	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
+	VM_BUG_ON_PAGE(!PageCompound(page), page);
 	page = compound_head(page);
 	return head_compound_pincount(page);
 }
@@ -913,7 +902,9 @@ static inline int compound_pincount(struct page *page)
 static inline void set_compound_order(struct page *page, unsigned int order)
 {
 	page[1].compound_order = order;
+#ifdef CONFIG_64BIT
 	page[1].compound_nr = 1U << order;
+#endif
 }
 
 /* Returns the number of pages in this potentially compound page. */
@@ -921,7 +912,11 @@ static inline unsigned long compound_nr(struct page *page)
 {
 	if (!PageHead(page))
 		return 1;
+#ifdef CONFIG_64BIT
 	return page[1].compound_nr;
+#else
+	return 1UL << compound_order(page);
+#endif
 }
 
 /* Returns the number of bytes in this potentially compound page. */
@@ -1269,7 +1264,7 @@ void unpin_user_pages(struct page **pages, unsigned long npages);
  */
 static inline bool page_maybe_dma_pinned(struct page *page)
 {
-	if (hpage_pincount_available(page))
+	if (PageCompound(page))
 		return compound_pincount(page) > 0;
 
 	/*

include/linux/mm_types.h

@@ -135,11 +135,14 @@ struct page {
 			unsigned char compound_dtor;
 			unsigned char compound_order;
 			atomic_t compound_mapcount;
+			atomic_t compound_pincount;
+#ifdef CONFIG_64BIT
 			unsigned int compound_nr; /* 1 << compound_order */
+#endif
 		};
 		struct {	/* Second tail page of compound page */
 			unsigned long _compound_pad_1;	/* compound_head */
-			atomic_t hpage_pinned_refcount;
+			unsigned long _compound_pad_2;
 			/* For both global and memcg */
 			struct list_head deferred_list;
 		};
@@ -300,7 +303,7 @@ static inline atomic_t *compound_mapcount_ptr(struct page *page)
 
 static inline atomic_t *compound_pincount_ptr(struct page *page)
 {
-	return &page[2].hpage_pinned_refcount;
+	return &page[1].compound_pincount;
 }
 
 /*

mm/debug.c

@@ -92,16 +92,10 @@ static void __dump_page(struct page *page)
 			page, page_ref_count(head), mapcount, mapping,
 			page_to_pgoff(page), page_to_pfn(page));
 	if (compound) {
-		if (hpage_pincount_available(page)) {
 		pr_warn("head:%p order:%u compound_mapcount:%d compound_pincount:%d\n",
 				head, compound_order(head),
 				head_compound_mapcount(head),
 				head_compound_pincount(head));
-		} else {
-			pr_warn("head:%p order:%u compound_mapcount:%d\n",
-					head, compound_order(head),
-					head_compound_mapcount(head));
-		}
 	}
 
 #ifdef CONFIG_MEMCG

mm/gup.c

@@ -99,12 +99,11 @@ retry:
  *
  *    FOLL_GET: page's refcount will be incremented by @refs.
  *
- *    FOLL_PIN on compound pages that are > two pages long: page's refcount will
- *    be incremented by @refs, and page[2].hpage_pinned_refcount will be
- *    incremented by @refs * GUP_PIN_COUNTING_BIAS.
+ *    FOLL_PIN on compound pages: page's refcount will be incremented by
+ *    @refs, and page[1].compound_pincount will be incremented by @refs.
  *
- *    FOLL_PIN on normal pages, or compound pages that are two pages long:
- *    page's refcount will be incremented by @refs * GUP_PIN_COUNTING_BIAS.
+ *    FOLL_PIN on normal pages: page's refcount will be incremented by
+ *    @refs * GUP_PIN_COUNTING_BIAS.
  *
  * Return: head page (with refcount appropriately incremented) for success, or
  * NULL upon failure. If neither FOLL_GET nor FOLL_PIN was set, that's
@@ -135,16 +134,15 @@ __maybe_unused struct page *try_grab_compound_head(struct page *page,
 			return NULL;
 
 		/*
-		 * When pinning a compound page of order > 1 (which is
-		 * what hpage_pincount_available() checks for), use an
-		 * exact count to track it.
+		 * When pinning a compound page, use an exact count to
+		 * track it.
 		 *
 		 * However, be sure to *also* increment the normal page
 		 * refcount field at least once, so that the page really
 		 * is pinned.  That's why the refcount from the earlier
 		 * try_get_compound_head() is left intact.
 		 */
-		if (hpage_pincount_available(page))
+		if (PageHead(page))
 			atomic_add(refs, compound_pincount_ptr(page));
 		else
 			page_ref_add(page, refs * (GUP_PIN_COUNTING_BIAS - 1));
@@ -166,7 +164,7 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
 	if (flags & FOLL_PIN) {
 		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED,
 				    refs);
-		if (hpage_pincount_available(page))
+		if (PageHead(page))
 			atomic_sub(refs, compound_pincount_ptr(page));
 		else
 			refs *= GUP_PIN_COUNTING_BIAS;
@@ -211,7 +209,7 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
 		 * increment the normal page refcount field at least once,
 		 * so that the page really is pinned.
 		 */
-		if (hpage_pincount_available(page)) {
+		if (PageHead(page)) {
 			page_ref_add(page, 1);
 			atomic_add(1, compound_pincount_ptr(page));
 		} else {

mm/hugetlb.c

@@ -1320,7 +1320,9 @@ static void __destroy_compound_gigantic_page(struct page *page,
 	}
 
 	set_compound_order(page, 0);
+#ifdef CONFIG_64BIT
 	page[1].compound_nr = 0;
+#endif
 	__ClearPageHead(page);
 }
 
@@ -1812,7 +1814,9 @@ out_error:
 	for (; j < nr_pages; j++, p = mem_map_next(p, page, j))
 		__ClearPageReserved(p);
 	set_compound_order(page, 0);
+#ifdef CONFIG_64BIT
 	page[1].compound_nr = 0;
+#endif
 	__ClearPageHead(page);
 	return false;
 }

mm/page_alloc.c

@@ -734,7 +734,6 @@ static void prep_compound_head(struct page *page, unsigned int order)
 	set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);
 	set_compound_order(page, order);
 	atomic_set(compound_mapcount_ptr(page), -1);
-	if (hpage_pincount_available(page))
 	atomic_set(compound_pincount_ptr(page), 0);
 }
 

mm/rmap.c

@@ -1219,7 +1219,6 @@ void page_add_new_anon_rmap(struct page *page,
 		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
 		/* increment count (starts at -1) */
 		atomic_set(compound_mapcount_ptr(page), 0);
-		if (hpage_pincount_available(page))
 		atomic_set(compound_pincount_ptr(page), 0);
 
 		__mod_lruvec_page_state(page, NR_ANON_THPS, nr);
@@ -2353,7 +2352,6 @@ void hugepage_add_new_anon_rmap(struct page *page,
 {
 	BUG_ON(address < vma->vm_start || address >= vma->vm_end);
 	atomic_set(compound_mapcount_ptr(page), 0);
-	if (hpage_pincount_available(page))
 	atomic_set(compound_pincount_ptr(page), 0);
 
 	__page_set_anon_rmap(page, vma, address, 1);