[PATCH v2 13/19] mm, swap: remove workaround for unsynchronized swap map cache state

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

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 cc2524e74120..adf6e33263f3 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -447,8 +447,6 @@ 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.
@@ -459,8 +457,7 @@ 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;
@@ -480,7 +477,7 @@ 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);
@@ -513,8 +510,6 @@ 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
@@ -527,8 +522,7 @@ 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;
@@ -549,8 +543,7 @@ 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
@@ -579,7 +572,7 @@ 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;
@@ -607,7 +600,7 @@ 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)
@@ -726,7 +719,7 @@ 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) {
@@ -744,7 +737,7 @@ 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;
@@ -849,7 +842,7 @@ 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)
@@ -871,7 +864,7 @@ 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;

-- 
2.51.2