[PATCH 04/10] mm: Remove PageSwapCache

Matthew Wilcox (Oracle)
2024-08-21T19:34:52Z

This flag is now only used on folios, so we can remove all the page
accessors and reword the comments that refer to them.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
include/linux/mm_types.h | 2 +-
include/linux/page-flags.h | 11 +++--------
mm/ksm.c | 19 ++++++++++---------
mm/migrate.c | 3 ++-
mm/shmem.c | 11 ++++++-----
5 files changed, 22 insertions(+), 24 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 2419e60c9a7f..6e3bdf8e38bc 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -109,7 +109,7 @@ struct page {
/**
* @private: Mapping-private opaque data.
* Usually used for buffer_heads if PagePrivate.
- * Used for swp_entry_t if PageSwapCache.
+ * Used for swp_entry_t if swapcache flag set.
* Indicates order in the buddy system if PageBuddy.
*/
unsigned long private;
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 2c2e6106682c..43a7996c53d4 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -574,15 +574,10 @@ static __always_inline bool folio_test_swapcache(const struct folio *folio)
test_bit(PG_swapcache, const_folio_flags(folio, 0));
}

-static __always_inline bool PageSwapCache(const struct page *page)
-{
- return folio_test_swapcache(page_folio(page));
-}
-
-SETPAGEFLAG(SwapCache, swapcache, PF_NO_TAIL)
-CLEARPAGEFLAG(SwapCache, swapcache, PF_NO_TAIL)
+FOLIO_SET_FLAG(swapcache, FOLIO_HEAD_PAGE)
+FOLIO_CLEAR_FLAG(swapcache, FOLIO_HEAD_PAGE)
#else
-PAGEFLAG_FALSE(SwapCache, swapcache)
+FOLIO_FLAG_FALSE(swapcache)
#endif

PAGEFLAG(Unevictable, unevictable, PF_HEAD)
diff --git a/mm/ksm.c b/mm/ksm.c
index 8e53666bc7b0..a2e2a521df0a 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -909,12 +909,13 @@ static struct folio *ksm_get_folio(struct ksm_stable_node *stable_node,
*/
while (!folio_try_get(folio)) {
/*
- * Another check for page->mapping != expected_mapping would
- * work here too. We have chosen the !PageSwapCache test to
- * optimize the common case, when the page is or is about to
- * be freed: PageSwapCache is cleared (under spin_lock_irq)
- * in the ref_freeze section of __remove_mapping(); but Anon
- * folio->mapping reset to NULL later, in free_pages_prepare().
+ * Another check for folio->mapping != expected_mapping
+ * would work here too. We have chosen to test the
+ * swapcache flag to optimize the common case, when the
+ * folio is or is about to be freed: the swapcache flag
+ * is cleared (under spin_lock_irq) in the ref_freeze
+ * section of __remove_mapping(); but anon folio->mapping
+ * is reset to NULL later, in free_pages_prepare().
*/
if (!folio_test_swapcache(folio))
goto stale;
@@ -945,7 +946,7 @@ static struct folio *ksm_get_folio(struct ksm_stable_node *stable_node,

stale:
/*
- * We come here from above when page->mapping or !PageSwapCache
+ * We come here from above when folio->mapping or the swapcache flag
* suggests that the node is stale; but it might be under migration.
* We need smp_rmb(), matching the smp_wmb() in folio_migrate_ksm(),
* before checking whether node->kpfn has been changed.
@@ -1452,7 +1453,7 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,
goto out;

/*
- * We need the page lock to read a stable PageSwapCache in
+ * We need the folio lock to read a stable swapcache flag in
* write_protect_page(). We use trylock_page() instead of
* lock_page() because we don't want to wait here - we
* prefer to continue scanning and merging different pages,
@@ -3123,7 +3124,7 @@ void folio_migrate_ksm(struct folio *newfolio, struct folio *folio)
* newfolio->mapping was set in advance; now we need smp_wmb()
* to make sure that the new stable_node->kpfn is visible
* to ksm_get_folio() before it can see that folio->mapping
- * has gone stale (or that folio_test_swapcache has been cleared).
+ * has gone stale (or that the swapcache flag has been cleared).
*/
smp_wmb();
folio_set_stable_node(folio, NULL);
diff --git a/mm/migrate.c b/mm/migrate.c
index 1248c89d4dbd..4f55f4930fe8 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -666,7 +666,8 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
folio_migrate_ksm(newfolio, folio);
/*
* Please do not reorder this without considering how mm/ksm.c's
- * ksm_get_folio() depends upon ksm_migrate_page() and PageSwapCache().
+ * ksm_get_folio() depends upon ksm_migrate_page() and the
+ * swapcache flag.
*/
if (folio_test_swapcache(folio))
folio_clear_swapcache(folio);
diff --git a/mm/shmem.c b/mm/shmem.c
index 22a3f3c1897e..752106aca845 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -502,8 +502,8 @@ static int shmem_replace_entry(struct address_space *mapping,
* Sometimes, before we decide whether to proceed or to fail, we must check
* that an entry was not already brought back from swap by a racing thread.
*
- * Checking page is not enough: by the time a SwapCache page is locked, it
- * might be reused, and again be SwapCache, using the same swap as before.
+ * Checking folio is not enough: by the time a swapcache folio is locked, it
+ * might be reused, and again be swapcache, using the same swap as before.
*/
static bool shmem_confirm_swap(struct address_space *mapping,
pgoff_t index, swp_entry_t swap)
@@ -1940,9 +1940,10 @@ static int shmem_replace_folio(struct folio **foliop, gfp_t gfp,

if (unlikely(error)) {
/*
- * Is this possible? I think not, now that our callers check
- * both PageSwapCache and page_private after getting page lock;
- * but be defensive. Reverse old to newpage for clear and free.
+ * Is this possible? I think not, now that our callers
+ * check both the swapcache flag and folio->private
+ * after getting the folio lock; but be defensive.
+ * Reverse old to newpage for clear and free.
*/
old = new;
} else {
--
2.43.0