From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
This gives us five words of space in a single union in struct page.
The compound_mapcount moves position (from offset 24 to offset 20)
on 64-bit systems, but that does not seem likely to cause any trouble.
Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
include/linux/mm_types.h | 97 +++++++++++++++++++---------------------
mm/page_alloc.c | 2 +-
2 files changed, 47 insertions(+), 52 deletions(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index b6a3948195d3..cf3bbee8c9a1 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -73,59 +73,19 @@ struct page {
unsigned long flags; /* Atomic flags, some possibly
* updated asynchronously */
/*
- * WARNING: bit 0 of the first word encode PageTail(). That means
- * the rest users of the storage space MUST NOT use the bit to
+ * Five words (20/40 bytes) are available in this union.
+ * WARNING: bit 0 of the first word is used for PageTail(). That
+ * means the other users of this union MUST NOT use the bit to
* avoid collision and false-positive PageTail().
*/
- union {
- struct list_head lru; /* Pageout list, eg. active_list
- * protected by zone_lru_lock !
- * Can be used as a generic list
- * by the page owner.
- */
- struct dev_pagemap *pgmap; /* ZONE_DEVICE pages are never on an
- * lru or handled by a slab
- * allocator, this points to the
- * hosting device page map.
- */
- struct { /* slub per cpu partial pages */
- struct page *next; /* Next partial slab */
-#ifdef CONFIG_64BIT
- int pages; /* Nr of partial slabs left */
- int pobjects; /* Approximate # of objects */
-#else
- short int pages;
- short int pobjects;
-#endif
- };
-
- struct rcu_head rcu_head; /* Used by SLAB
- * when destroying via RCU
- */
- /* Tail pages of compound page */
- struct {
- unsigned long compound_head; /* If bit zero is set */
-
- /* First tail page only */
- unsigned char compound_dtor;
- unsigned char compound_order;
- /* two/six bytes available here */
- };
-
-#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && USE_SPLIT_PMD_PTLOCKS
- struct {
- unsigned long __pad; /* do not overlay pmd_huge_pte
- * with compound_head to avoid
- * possible bit 0 collision.
- */
- pgtable_t pmd_huge_pte; /* protected by page->ptl */
- };
-#endif
- };
-
- /* Three words (12/24 bytes) are available in this union. */
union {
struct { /* Page cache and anonymous pages */
+ /**
+ * @lru: Pageout list, eg. active_list protected by
+ * zone_lru_lock. Sometimes used as a generic list
+ * by the page owner.
+ */
+ struct list_head lru;
/* See page-flags.h for PAGE_MAPPING_FLAGS */
struct address_space *mapping;
pgoff_t index; /* Our offset within mapping. */
@@ -138,6 +98,19 @@ struct page {
unsigned long private;
};
struct { /* slab, slob and slub */
+ union {
+ struct list_head slab_list; /* uses lru */
+ struct { /* Partial pages */
+ struct page *next;
+#ifdef CONFIG_64BIT
+ int pages; /* Nr of pages left */
+ int pobjects; /* Approximate count */
+#else
+ short int pages;
+ short int pobjects;
+#endif
+ };
+ };
struct kmem_cache *slab_cache; /* not slob */
/* Double-word boundary */
void *freelist; /* first free object */
@@ -151,9 +124,22 @@ struct page {
};
};
};
- atomic_t compound_mapcount; /* first tail page */
- struct list_head deferred_list; /* second tail page */
+ struct { /* Tail pages of compound page */
+ unsigned long compound_head; /* Bit zero is set */
+
+ /* First tail page only */
+ unsigned char compound_dtor;
+ unsigned char compound_order;
+ atomic_t compound_mapcount;
+ };
+ struct { /* Second tail page of compound page */
+ unsigned long _compound_pad_1; /* compound_head */
+ unsigned long _compound_pad_2;
+ struct list_head deferred_list;
+ };
struct { /* Page table pages */
+ unsigned long _pt_pad_1; /* compound_head */
+ pgtable_t pmd_huge_pte; /* protected by page->ptl */
unsigned long _pt_pad_2; /* mapping */
unsigned long _pt_pad_3;
#if ALLOC_SPLIT_PTLOCKS
@@ -162,6 +148,15 @@ struct page {
spinlock_t ptl;
#endif
};
+
+ /** @rcu_head: You can use this to free a page by RCU. */
+ struct rcu_head rcu_head;
+
+ /**
+ * @pgmap: For ZONE_DEVICE pages, this points to the hosting
+ * device page map.
+ */
+ struct dev_pagemap *pgmap;
};
union { /* This union is 4 bytes in size. */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1a0149c4f672..787440218def 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -924,7 +924,7 @@ static int free_tail_pages_check(struct page *head_page, struct page *page)
}
switch (page - head_page) {
case 1:
- /* the first tail page: ->mapping is compound_mapcount() */
+ /* the first tail page: ->mapping may be compound_mapcount() */
if (unlikely(compound_mapcount(page))) {
bad_page(page, "nonzero compound_mapcount", 0);
goto out;
--
2.17.0
