[PATCH v4 13/19] mm, swap: remove workaround for unsynchronized swap map cache state

Kairui Song
2025-12-04T19:30:42Z

Remove the "skip if exists" check from commit a65b0e7607ccb ("zswap:
make shrinking memcg-aware"). It was needed because there is a tiny time
window between setting the SWAP_HAS_CACHE bit and actually adding the
folio to the swap cache. If a user is trying to add the folio into the
swap cache but another user was interrupted after setting SWAP_HAS_CACHE
but hasn't added the folio to the swap cache yet, it might lead to a
deadlock.

We have moved the bit setting to the same critical section as adding the
folio, so this is no longer needed. Remove it and clean it up.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
mm/swap.h | 2 +-
mm/swap_state.c | 27 ++++++++++-----------------
mm/zswap.c | 2 +-
3 files changed, 12 insertions(+), 19 deletions(-)

diff --git a/mm/swap.h b/mm/swap.h
index b5075a1aee04..6777b2ab9d92 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -260,7 +260,7 @@ int swap_cache_add_folio(struct folio *folio, swp_entry_t entry,
void swap_cache_del_folio(struct folio *folio);
struct folio *swap_cache_alloc_folio(swp_entry_t entry, gfp_t gfp_flags,
struct mempolicy *mpol, pgoff_t ilx,
- bool *alloced, bool skip_if_exists);
+ bool *alloced);
/* Below helpers require the caller to lock and pass in the swap cluster. */
void __swap_cache_del_folio(struct swap_cluster_info *ci,
struct folio *folio, swp_entry_t entry, void *shadow);
diff --git a/mm/swap_state.c b/mm/swap_state.c
index df7df8b75e52..1a69ba3be87f 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -445,8 +445,6 @@ void swap_update_readahead(struct folio *folio, struct vm_area_struct *vma,
* @folio: folio to be added.
* @gfp: memory allocation flags for charge, can be 0 if @charged if true.
* @charged: if the folio is already charged.
- * @skip_if_exists: if the slot is in a cached state, return NULL.
- * This is an old workaround that will be removed shortly.
*
* Update the swap_map and add folio as swap cache, typically before swapin.
* All swap slots covered by the folio must have a non-zero swap count.
@@ -457,8 +455,7 @@ void swap_update_readahead(struct folio *folio, struct vm_area_struct *vma,
*/
static struct folio *__swap_cache_prepare_and_add(swp_entry_t entry,
struct folio *folio,
- gfp_t gfp, bool charged,
- bool skip_if_exists)
+ gfp_t gfp, bool charged)
{
struct folio *swapcache = NULL;
void *shadow;
@@ -478,7 +475,7 @@ static struct folio *__swap_cache_prepare_and_add(swp_entry_t entry,
* might return a folio that is irrelevant to the faulting
* entry because @entry is aligned down. Just return NULL.
*/
- if (ret != -EEXIST || skip_if_exists || folio_test_large(folio))
+ if (ret != -EEXIST || folio_test_large(folio))
goto failed;

swapcache = swap_cache_get_folio(entry);
@@ -511,8 +508,6 @@ static struct folio *__swap_cache_prepare_and_add(swp_entry_t entry,
* @mpol: NUMA memory allocation policy to be applied
* @ilx: NUMA interleave index, for use only when MPOL_INTERLEAVE
* @new_page_allocated: sets true if allocation happened, false otherwise
- * @skip_if_exists: if the slot is a partially cached state, return NULL.
- * This is a workaround that would be removed shortly.
*
* Allocate a folio in the swap cache for one swap slot, typically before
* doing IO (e.g. swap in or zswap writeback). The swap slot indicated by
@@ -525,8 +520,7 @@ static struct folio *__swap_cache_prepare_and_add(swp_entry_t entry,
*/
struct folio *swap_cache_alloc_folio(swp_entry_t entry, gfp_t gfp_mask,
struct mempolicy *mpol, pgoff_t ilx,
- bool *new_page_allocated,
- bool skip_if_exists)
+ bool *new_page_allocated)
{
struct swap_info_struct *si = __swap_entry_to_info(entry);
struct folio *folio;
@@ -547,8 +541,7 @@ struct folio *swap_cache_alloc_folio(swp_entry_t entry, gfp_t gfp_mask,
if (!folio)
return NULL;
/* Try add the new folio, returns existing folio or NULL on failure. */
- result = __swap_cache_prepare_and_add(entry, folio, gfp_mask,
- false, skip_if_exists);
+ result = __swap_cache_prepare_and_add(entry, folio, gfp_mask, false);
if (result == folio)
*new_page_allocated = true;
else
@@ -577,7 +570,7 @@ struct folio *swapin_folio(swp_entry_t entry, struct folio *folio)
unsigned long nr_pages = folio_nr_pages(folio);

entry = swp_entry(swp_type(entry), round_down(offset, nr_pages));
- swapcache = __swap_cache_prepare_and_add(entry, folio, 0, true, false);
+ swapcache = __swap_cache_prepare_and_add(entry, folio, 0, true);
if (swapcache == folio)
swap_read_folio(folio, NULL);
return swapcache;
@@ -605,7 +598,7 @@ struct folio *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,

mpol = get_vma_policy(vma, addr, 0, &ilx);
folio = swap_cache_alloc_folio(entry, gfp_mask, mpol, ilx,
- &page_allocated, false);
+ &page_allocated);
mpol_cond_put(mpol);

if (page_allocated)
@@ -724,7 +717,7 @@ struct folio *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
/* Ok, do the async read-ahead now */
folio = swap_cache_alloc_folio(
swp_entry(swp_type(entry), offset), gfp_mask, mpol, ilx,
- &page_allocated, false);
+ &page_allocated);
if (!folio)
continue;
if (page_allocated) {
@@ -742,7 +735,7 @@ struct folio *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
skip:
/* The page was likely read above, so no need for plugging here */
folio = swap_cache_alloc_folio(entry, gfp_mask, mpol, ilx,
- &page_allocated, false);
+ &page_allocated);
if (unlikely(page_allocated))
swap_read_folio(folio, NULL);
return folio;
@@ -847,7 +840,7 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
continue;
}
folio = swap_cache_alloc_folio(entry, gfp_mask, mpol, ilx,
- &page_allocated, false);
+ &page_allocated);
if (si)
put_swap_device(si);
if (!folio)
@@ -869,7 +862,7 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
skip:
/* The folio was likely read above, so no need for plugging here */
folio = swap_cache_alloc_folio(targ_entry, gfp_mask, mpol, targ_ilx,
- &page_allocated, false);
+ &page_allocated);
if (unlikely(page_allocated))
swap_read_folio(folio, NULL);
return folio;
diff --git a/mm/zswap.c b/mm/zswap.c
index a7a2443912f4..d8a33db9d3cc 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1015,7 +1015,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,

mpol = get_task_policy(current);
folio = swap_cache_alloc_folio(swpentry, GFP_KERNEL, mpol,
- NO_INTERLEAVE_INDEX, &folio_was_allocated, true);
+ NO_INTERLEAVE_INDEX, &folio_was_allocated);
put_swap_device(si);
if (!folio)
return -ENOMEM;

--
2.52.0