This is a preparatory patch. __swap_cache_alloc_folio() adds the new folio to the LRU itself, which leaves its callers no way to act on the folio before it becomes visible to reclaim. Two users need exactly that: - the next patch moves the refault evaluation out of the swap cache folio allocation, and it has to happen before folio_add_lru(): that consumes PG_active to file the folio on the inactive or the active list, and under MGLRU it also reads PG_workingset to pick the generation. Setting either flag afterwards does not move the folio; - the upcoming zswap writeback dropbehind implementation needs the buffer folio to stay off the LRU entirely, as the per-CPU LRU batch would hold a reference on it and keep remove_mapping() from freeing it once writeback completes. Defer the LRU insertion to the callers of __swap_cache_alloc_folio(): each of them adds the folio right after the allocation, so there is no functional change intended. Suggested-by: Kairui Song Signed-off-by: Alexandre Ghiti --- mm/swap.h | 6 +++--- mm/swap_state.c | 20 ++++++++++++-------- mm/zswap.c | 5 +++-- 3 files changed, 18 insertions(+), 13 deletions(-) diff --git a/mm/swap.h b/mm/swap.h index 90a551a88df6..8679cb61268e 100644 --- a/mm/swap.h +++ b/mm/swap.h @@ -312,9 +312,9 @@ bool swap_cache_has_folio(swp_entry_t entry); struct folio *swap_cache_get_folio(swp_entry_t entry); void *swap_cache_get_shadow(swp_entry_t entry); void swap_cache_del_folio(struct folio *folio); -struct folio *swap_cache_alloc_folio(swp_entry_t target_entry, gfp_t gfp_mask, - unsigned long orders, struct vm_fault *vmf, - struct mempolicy *mpol, pgoff_t ilx); +struct folio *__swap_cache_alloc_folio(swp_entry_t target_entry, gfp_t gfp_mask, + unsigned long orders, struct vm_fault *vmf, + struct mempolicy *mpol, pgoff_t ilx); /* Below helpers require the caller to lock and pass in the swap cluster. */ void __swap_cache_add_folio(struct swap_cluster_info *ci, struct folio *folio, swp_entry_t entry); diff --git a/mm/swap_state.c b/mm/swap_state.c index b76eb3d876fd..bf8ff2d2dbf1 100644 --- a/mm/swap_state.c +++ b/mm/swap_state.c @@ -489,13 +489,11 @@ static struct folio *__swap_cache_alloc(struct swap_cluster_info *ci, node_stat_mod_folio(folio, NR_FILE_PAGES, nr_pages); lruvec_stat_mod_folio(folio, NR_SWAPCACHE, nr_pages); - /* Caller will initiate read into locked new_folio */ - folio_add_lru(folio); return folio; } /** - * swap_cache_alloc_folio - Allocate folio for swapped out slot in swap cache. + * __swap_cache_alloc_folio - Allocate folio for swapped out slot in swap cache. * @targ_entry: swap entry indicating the target slot * @gfp: memory allocation flags * @orders: allocation orders, must be non zero @@ -507,13 +505,17 @@ static struct folio *__swap_cache_alloc(struct swap_cluster_info *ci, * doing IO (e.g. swap in or zswap writeback). The swap slot indicated by * @targ_entry must have a non-zero swap count (swapped out). * + * The returned folio is locked and is NOT on the LRU. The caller must either + * add it to the LRU with folio_add_lru() so page reclaim can find it, or free + * it directly once done; a folio left off the LRU is unreclaimable and leaks. + * * Context: Caller must protect the swap device with reference count or locks. * Return: Returns the folio if allocation succeeded and folio is in the swap * cache. Returns error code if failed due to race, OOM or invalid arguments. */ -struct folio *swap_cache_alloc_folio(swp_entry_t targ_entry, gfp_t gfp, - unsigned long orders, struct vm_fault *vmf, - struct mempolicy *mpol, pgoff_t ilx) +struct folio *__swap_cache_alloc_folio(swp_entry_t targ_entry, gfp_t gfp, + unsigned long orders, struct vm_fault *vmf, + struct mempolicy *mpol, pgoff_t ilx) { int order, err; struct folio *ret; @@ -649,12 +651,13 @@ static struct folio *swap_cache_read_folio(struct swap_io_ctx *ctx, folio = swap_cache_get_folio(entry); if (folio) return folio; - folio = swap_cache_alloc_folio(entry, gfp, BIT(0), NULL, mpol, ilx); + folio = __swap_cache_alloc_folio(entry, gfp, BIT(0), NULL, mpol, ilx); } while (PTR_ERR(folio) == -EEXIST); if (IS_ERR_OR_NULL(folio)) return NULL; + folio_add_lru(folio); swap_read_folio(ctx, folio); if (readahead) { folio_set_readahead(folio); @@ -690,12 +693,13 @@ struct folio *swapin_sync(swp_entry_t entry, gfp_t gfp, unsigned long orders, folio = swap_cache_get_folio(entry); if (folio) return folio; - folio = swap_cache_alloc_folio(entry, gfp, orders, vmf, mpol, ilx); + folio = __swap_cache_alloc_folio(entry, gfp, orders, vmf, mpol, ilx); } while (PTR_ERR(folio) == -EEXIST); if (IS_ERR(folio)) return folio; + folio_add_lru(folio); swap_read_folio(&ctx, folio); swap_read_submit(&ctx); return folio; diff --git a/mm/zswap.c b/mm/zswap.c index 37f34e406c8e..0d2efe21f18a 100644 --- a/mm/zswap.c +++ b/mm/zswap.c @@ -1001,8 +1001,8 @@ static int zswap_writeback_entry(struct zswap_entry *entry, return -EEXIST; mpol = get_task_policy(current); - folio = swap_cache_alloc_folio(swpentry, GFP_KERNEL, BIT(0), NULL, mpol, - NO_INTERLEAVE_INDEX); + folio = __swap_cache_alloc_folio(swpentry, GFP_KERNEL, BIT(0), NULL, mpol, + NO_INTERLEAVE_INDEX); put_swap_device(si); /* @@ -1014,6 +1014,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry, */ if (IS_ERR(folio)) return PTR_ERR(folio); + folio_add_lru(folio); /* * folio is locked, and the swapcache is now secured against -- 2.53.0-Meta