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>
---
include/linux/mm_types.h | 96 ++++++++++++++++++----------------------
mm/page_alloc.c | 2 +-
2 files changed, 45 insertions(+), 53 deletions(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 230d473f16da..080ea97ad444 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -73,58 +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
- };
-
- union { /* This union is three words (12/24 bytes) in size */
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. */
@@ -137,11 +98,20 @@ struct page {
unsigned long private;
};
struct { /* slab and slob */
+ struct list_head slab_list;
struct kmem_cache *slab_cache;
void *freelist; /* first free object */
void *s_mem; /* first object */
};
struct { /* slub also uses some of the slab fields */
+ 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 kmem_cache *slub_cache;
/* Double-word boundary */
void *slub_freelist;
@@ -149,17 +119,39 @@ struct page {
unsigned objects:15;
unsigned frozen:1;
};
- 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 _ptl_pad_1;
- unsigned long _ptl_pad_2;
+ unsigned long _pt_pad_1; /* compound_head */
+ pgtable_t pmd_huge_pte; /* protected by page->ptl */
+ unsigned long _pt_pad_2;
+ unsigned long _pt_pad_3;
#if ALLOC_SPLIT_PTLOCKS
spinlock_t *ptl;
#else
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 18720eccbce1..d1e4df7d57bf 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -944,7 +944,7 @@ 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
