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Sun, 16 Nov 2025 10:12:48 -0800 (PST) Received: from [127.0.0.1] ([101.32.222.185]) by smtp.gmail.com with ESMTPSA id af79cd13be357-8b2e089b436sm305447785a.45.2025.11.16.10.12.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 16 Nov 2025 10:12:47 -0800 (PST) From: Kairui Song Date: Mon, 17 Nov 2025 02:11:49 +0800 Subject: [PATCH v2 08/19] mm/shmem, swap: remove SWAP_MAP_SHMEM MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20251117-swap-table-p2-v2-8-37730e6ea6d5@tencent.com> References: <20251117-swap-table-p2-v2-0-37730e6ea6d5@tencent.com> In-Reply-To: <20251117-swap-table-p2-v2-0-37730e6ea6d5@tencent.com> To: linux-mm@kvack.org Cc: Andrew Morton , Baoquan He , Barry Song , Chris Li , Nhat Pham , Yosry Ahmed , David Hildenbrand , Johannes Weiner , Youngjun Park , Hugh Dickins , Baolin Wang , Ying Huang , Kemeng Shi , Lorenzo Stoakes , "Matthew Wilcox (Oracle)" , linux-kernel@vger.kernel.org, Kairui Song X-Mailer: b4 0.14.3 X-Developer-Signature: v=1; a=ed25519-sha256; t=1763316712; l=7126; i=kasong@tencent.com; s=kasong-sign-tencent; h=from:subject:message-id; bh=dny+SXWuyE8K8IiOt2sIszvBbGvpqP/hk6etEBD5hKg=; b=fV49ZBo98ttH/vcQHSgk1G7A+xp6sTNohz8hLqQKodxZHVkd6sIjzP90SLf+Fx591oGAtVZQ/ xppHv2AvP0VCwBuc/2MuqSCo0w8KO7WyaIl7VrHNfunmODkQu/auvAa X-Developer-Key: i=kasong@tencent.com; a=ed25519; pk=kCdoBuwrYph+KrkJnrr7Sm1pwwhGDdZKcKrqiK8Y1mI= X-Rspamd-Queue-Id: 0A349A0012 X-Rspamd-Server: rspam07 X-Stat-Signature: 8eib7rdrseanmadpgktpdjs69uxpmoj9 X-Rspam-User: X-HE-Tag: 1763316768-518108 X-HE-Meta: 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 fuS3zJn0 DBusJfn38g88jZpMPTGNX+VXyikKhjMlmvKUtdTbeJf2BKMwd5ahXjCvvmMrNxLZ6ZuAt34BkcbZD/T0zn/VPObtaguilROsGexEMIVWsbd8mYobZ4b11FI1zC/bGWy/xp2DQRZ+/oxNLToBGR5vUOvTXC5s61njUdy9FoODJswMIpqmaZBI/dvPVMlab14GH+qD6z5vF0OflGbyklENc1Sjz0BRADq/uW+LZPT+7hI3uHCfdzOyPjZoBC5SfBXwzGm5UOGko57e8mosdpHzfh57Vh2m2INuKyDa1PW08RGSfOje0Z6tiHXBPSmXnn4cSkY6a0WtqdJa7hvmXD42YQevElX6+g4PYmSNKMBZWO2SwFZMMUEGxaSC2XITqSy6ZhwYflyZKebhbNymx/edviXE6vTkS1c/zyavZSXITHCWmtQJs4SXG+SFtpxVMn0tgDkZdeioz58Y3288= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Nhat Pham The SWAP_MAP_SHMEM state was introduced in the commit aaa468653b4a ("swap_info: note SWAP_MAP_SHMEM"), to quickly determine if a swap entry belongs to shmem during swapoff. However, swapoff has since been rewritten in the commit b56a2d8af914 ("mm: rid swapoff of quadratic complexity"). Now having swap count == SWAP_MAP_SHMEM value is basically the same as having swap count == 1, and swap_shmem_alloc() behaves analogously to swap_duplicate(). The only difference of note is that swap_shmem_alloc() does not check for -ENOMEM returned from __swap_duplicate(), but it is OK because shmem never re-duplicates any swap entry it owns. This will stil be safe if we use (batched) swap_duplicate() instead. This commit adds swap_duplicate_nr(), the batched variant of swap_duplicate(), and removes the SWAP_MAP_SHMEM state and the associated swap_shmem_alloc() helper to simplify the state machine (both mentally and in terms of actual code). We will also have an extra state/special value that can be repurposed (for swap entries that never gets re-duplicated). Signed-off-by: Nhat Pham Signed-off-by: Kairui Song --- include/linux/swap.h | 15 +++++++-------- mm/shmem.c | 2 +- mm/swapfile.c | 42 +++++++++++++++++------------------------- 3 files changed, 25 insertions(+), 34 deletions(-) diff --git a/include/linux/swap.h b/include/linux/swap.h index 38ca3df68716..bf72b548a96d 100644 --- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -230,7 +230,6 @@ enum { /* Special value in first swap_map */ #define SWAP_MAP_MAX 0x3e /* Max count */ #define SWAP_MAP_BAD 0x3f /* Note page is bad */ -#define SWAP_MAP_SHMEM 0xbf /* Owned by shmem/tmpfs */ /* Special value in each swap_map continuation */ #define SWAP_CONT_MAX 0x7f /* Max count */ @@ -458,8 +457,7 @@ bool folio_free_swap(struct folio *folio); void put_swap_folio(struct folio *folio, swp_entry_t entry); extern swp_entry_t get_swap_page_of_type(int); extern int add_swap_count_continuation(swp_entry_t, gfp_t); -extern void swap_shmem_alloc(swp_entry_t, int); -extern int swap_duplicate(swp_entry_t); +extern int swap_duplicate_nr(swp_entry_t entry, int nr); extern int swapcache_prepare(swp_entry_t entry, int nr); extern void swap_free_nr(swp_entry_t entry, int nr_pages); extern void free_swap_and_cache_nr(swp_entry_t entry, int nr); @@ -514,11 +512,7 @@ static inline int add_swap_count_continuation(swp_entry_t swp, gfp_t gfp_mask) return 0; } -static inline void swap_shmem_alloc(swp_entry_t swp, int nr) -{ -} - -static inline int swap_duplicate(swp_entry_t swp) +static inline int swap_duplicate_nr(swp_entry_t swp, int nr_pages) { return 0; } @@ -569,6 +563,11 @@ static inline int add_swap_extent(struct swap_info_struct *sis, } #endif /* CONFIG_SWAP */ +static inline int swap_duplicate(swp_entry_t entry) +{ + return swap_duplicate_nr(entry, 1); +} + static inline void free_swap_and_cache(swp_entry_t entry) { free_swap_and_cache_nr(entry, 1); diff --git a/mm/shmem.c b/mm/shmem.c index 8b711b8bcdba..0b0af71ce5e8 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1654,7 +1654,7 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug, spin_unlock(&shmem_swaplist_lock); } - swap_shmem_alloc(folio->swap, nr_pages); + swap_duplicate_nr(folio->swap, nr_pages); shmem_delete_from_page_cache(folio, swp_to_radix_entry(folio->swap)); BUG_ON(folio_mapped(folio)); diff --git a/mm/swapfile.c b/mm/swapfile.c index a433445e9225..580adf9d93bd 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -201,7 +201,7 @@ static bool swap_is_last_map(struct swap_info_struct *si, unsigned char *map_end = map + nr_pages; unsigned char count = *map; - if (swap_count(count) != 1 && swap_count(count) != SWAP_MAP_SHMEM) + if (swap_count(count) != 1) return false; while (++map < map_end) { @@ -1522,12 +1522,6 @@ static unsigned char swap_entry_put_locked(struct swap_info_struct *si, if (usage == SWAP_HAS_CACHE) { VM_BUG_ON(!has_cache); has_cache = 0; - } else if (count == SWAP_MAP_SHMEM) { - /* - * Or we could insist on shmem.c using a special - * swap_shmem_free() and free_shmem_swap_and_cache()... - */ - count = 0; } else if ((count & ~COUNT_CONTINUED) <= SWAP_MAP_MAX) { if (count == COUNT_CONTINUED) { if (swap_count_continued(si, offset, count)) @@ -1625,7 +1619,7 @@ static bool swap_entries_put_map(struct swap_info_struct *si, if (nr <= 1) goto fallback; count = swap_count(data_race(si->swap_map[offset])); - if (count != 1 && count != SWAP_MAP_SHMEM) + if (count != 1) goto fallback; ci = swap_cluster_lock(si, offset); @@ -1679,12 +1673,10 @@ static bool swap_entries_put_map_nr(struct swap_info_struct *si, /* * Check if it's the last ref of swap entry in the freeing path. - * Qualified value includes 1, SWAP_HAS_CACHE or SWAP_MAP_SHMEM. */ static inline bool __maybe_unused swap_is_last_ref(unsigned char count) { - return (count == SWAP_HAS_CACHE) || (count == 1) || - (count == SWAP_MAP_SHMEM); + return (count == SWAP_HAS_CACHE) || (count == 1); } /* @@ -3678,7 +3670,6 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr) offset = swp_offset(entry); VM_WARN_ON(nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER); - VM_WARN_ON(usage == 1 && nr > 1); ci = swap_cluster_lock(si, offset); err = 0; @@ -3738,27 +3729,28 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr) return err; } -/* - * Help swapoff by noting that swap entry belongs to shmem/tmpfs - * (in which case its reference count is never incremented). - */ -void swap_shmem_alloc(swp_entry_t entry, int nr) -{ - __swap_duplicate(entry, SWAP_MAP_SHMEM, nr); -} - -/* - * Increase reference count of swap entry by 1. +/** + * swap_duplicate_nr() - Increase reference count of nr contiguous swap entries + * by 1. + * + * @entry: first swap entry from which we want to increase the refcount. + * @nr: Number of entries in range. + * * Returns 0 for success, or -ENOMEM if a swap_count_continuation is required * but could not be atomically allocated. Returns 0, just as if it succeeded, * if __swap_duplicate() fails for another reason (-EINVAL or -ENOENT), which * might occur if a page table entry has got corrupted. + * + * Note that we are currently not handling the case where nr > 1 and we need to + * add swap count continuation. This is OK, because no such user exists - shmem + * is the only user that can pass nr > 1, and it never re-duplicates any swap + * entry it owns. */ -int swap_duplicate(swp_entry_t entry) +int swap_duplicate_nr(swp_entry_t entry, int nr) { int err = 0; - while (!err && __swap_duplicate(entry, 1, 1) == -ENOMEM) + while (!err && __swap_duplicate(entry, 1, nr) == -ENOMEM) err = add_swap_count_continuation(entry, GFP_ATOMIC); return err; } -- 2.51.2