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Sun, 16 Nov 2025 10:13:38 -0800 (PST) From: Kairui Song Date: Mon, 17 Nov 2025 02:11:57 +0800 Subject: [PATCH v2 16/19] mm, swap: check swap table directly for checking cache Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20251117-swap-table-p2-v2-16-37730e6ea6d5@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=7942; i=kasong@tencent.com; s=kasong-sign-tencent; h=from:subject:message-id; bh=4LpI0+nubS4oB/IzKKwW7+HHeGQMbTZwqJ7RekkxmGo=; b=BfhHqraQQgddSnRblDBV/DhFJETHAshg2hqQyk3fx7cdQg6LZuyml4vb9T0EXG0WRz2Evddtw 3pcvF03q2E6B+CRONJEprqksBeE7MXyl+UW8rdD6kVZm7KGKnKq/EMu X-Developer-Key: i=kasong@tencent.com; a=ed25519; pk=kCdoBuwrYph+KrkJnrr7Sm1pwwhGDdZKcKrqiK8Y1mI= From: Kairui Song Instead of looking at the swap map, check swap table directly to tell if a swap slot is cached. Prepares for the removal of SWAP_HAS_CACHE. Signed-off-by: Kairui Song --- mm/swap.h | 11 ++++++++--- mm/swap_state.c | 16 ++++++++++++++++ mm/swapfile.c | 55 +++++++++++++++++++++++++++++-------------------------- mm/userfaultfd.c | 10 +++------- 4 files changed, 56 insertions(+), 36 deletions(-) diff --git a/mm/swap.h b/mm/swap.h index ec1ef7d0c35b..3692e143eeba 100644 --- a/mm/swap.h +++ b/mm/swap.h @@ -275,6 +275,7 @@ void __swapcache_clear_cached(struct swap_info_struct *si, * swap entries in the page table, similar to locking swap cache folio. * - See the comment of get_swap_device() for more complex usage. */ +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); @@ -335,8 +336,6 @@ static inline int swap_zeromap_batch(swp_entry_t entry, int max_nr, static inline int non_swapcache_batch(swp_entry_t entry, int max_nr) { - struct swap_info_struct *si = __swap_entry_to_info(entry); - pgoff_t offset = swp_offset(entry); int i; /* @@ -345,8 +344,9 @@ static inline int non_swapcache_batch(swp_entry_t entry, int max_nr) * be in conflict with the folio in swap cache. */ for (i = 0; i < max_nr; i++) { - if ((si->swap_map[offset + i] & SWAP_HAS_CACHE)) + if (swap_cache_has_folio(entry)) return i; + entry.val++; } return i; @@ -449,6 +449,11 @@ static inline int swap_writeout(struct folio *folio, return 0; } +static inline bool swap_cache_has_folio(swp_entry_t entry) +{ + return false; +} + static inline struct folio *swap_cache_get_folio(swp_entry_t entry) { return NULL; diff --git a/mm/swap_state.c b/mm/swap_state.c index ca1b7954bbb8..e9ae7c09c2bf 100644 --- a/mm/swap_state.c +++ b/mm/swap_state.c @@ -103,6 +103,22 @@ struct folio *swap_cache_get_folio(swp_entry_t entry) return NULL; } +/** + * swap_cache_has_folio - Check if a swap slot has cache. + * @entry: swap entry indicating the slot. + * + * Context: Caller must ensure @entry is valid and protect the swap + * device with reference count or locks. + */ +bool swap_cache_has_folio(swp_entry_t entry) +{ + unsigned long swp_tb; + + swp_tb = swap_table_get(__swap_entry_to_cluster(entry), + swp_cluster_offset(entry)); + return swp_tb_is_folio(swp_tb); +} + /** * swap_cache_get_shadow - Looks up a shadow in the swap cache. * @entry: swap entry used for the lookup. diff --git a/mm/swapfile.c b/mm/swapfile.c index ea18096444d7..e321093d0552 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -788,23 +788,18 @@ static unsigned int cluster_reclaim_range(struct swap_info_struct *si, unsigned int nr_pages = 1 << order; unsigned long offset = start, end = start + nr_pages; unsigned char *map = si->swap_map; - int nr_reclaim; + unsigned long swp_tb; spin_unlock(&ci->lock); do { - switch (READ_ONCE(map[offset])) { - case 0: + if (swap_count(READ_ONCE(map[offset]))) break; - case SWAP_HAS_CACHE: - nr_reclaim = __try_to_reclaim_swap(si, offset, TTRS_ANYWAY); - if (nr_reclaim < 0) - goto out; - break; - default: - goto out; + swp_tb = swap_table_get(ci, offset % SWAPFILE_CLUSTER); + if (swp_tb_is_folio(swp_tb)) { + if (__try_to_reclaim_swap(si, offset, TTRS_ANYWAY) < 0) + break; } } while (++offset < end); -out: spin_lock(&ci->lock); /* @@ -820,37 +815,41 @@ static unsigned int cluster_reclaim_range(struct swap_info_struct *si, * Recheck the range no matter reclaim succeeded or not, the slot * could have been be freed while we are not holding the lock. */ - for (offset = start; offset < end; offset++) - if (READ_ONCE(map[offset])) + for (offset = start; offset < end; offset++) { + swp_tb = __swap_table_get(ci, offset % SWAPFILE_CLUSTER); + if (swap_count(map[offset]) || !swp_tb_is_null(swp_tb)) return SWAP_ENTRY_INVALID; + } return start; } static bool cluster_scan_range(struct swap_info_struct *si, struct swap_cluster_info *ci, - unsigned long start, unsigned int nr_pages, + unsigned long offset, unsigned int nr_pages, bool *need_reclaim) { - unsigned long offset, end = start + nr_pages; + unsigned long end = offset + nr_pages; unsigned char *map = si->swap_map; + unsigned long swp_tb; if (cluster_is_empty(ci)) return true; - for (offset = start; offset < end; offset++) { - switch (READ_ONCE(map[offset])) { - case 0: - continue; - case SWAP_HAS_CACHE: + do { + if (swap_count(map[offset])) + return false; + swp_tb = __swap_table_get(ci, offset % SWAPFILE_CLUSTER); + if (swp_tb_is_folio(swp_tb)) { + WARN_ON_ONCE(!(map[offset] & SWAP_HAS_CACHE)); if (!vm_swap_full()) return false; *need_reclaim = true; - continue; - default: - return false; + } else { + /* A entry with no count and no cache must be null */ + VM_WARN_ON_ONCE(!swp_tb_is_null(swp_tb)); } - } + } while (++offset < end); return true; } @@ -1015,7 +1014,8 @@ static void swap_reclaim_full_clusters(struct swap_info_struct *si, bool force) to_scan--; while (offset < end) { - if (READ_ONCE(map[offset]) == SWAP_HAS_CACHE) { + if (!swap_count(READ_ONCE(map[offset])) && + swp_tb_is_folio(__swap_table_get(ci, offset % SWAPFILE_CLUSTER))) { spin_unlock(&ci->lock); nr_reclaim = __try_to_reclaim_swap(si, offset, TTRS_ANYWAY); @@ -1958,6 +1958,7 @@ void swap_put_entries_direct(swp_entry_t entry, int nr) struct swap_info_struct *si; bool any_only_cache = false; unsigned long offset; + unsigned long swp_tb; si = get_swap_device(entry); if (WARN_ON_ONCE(!si)) @@ -1982,7 +1983,9 @@ void swap_put_entries_direct(swp_entry_t entry, int nr) */ for (offset = start_offset; offset < end_offset; offset += nr) { nr = 1; - if (READ_ONCE(si->swap_map[offset]) == SWAP_HAS_CACHE) { + swp_tb = swap_table_get(__swap_offset_to_cluster(si, offset), + offset % SWAPFILE_CLUSTER); + if (!swap_count(READ_ONCE(si->swap_map[offset])) && swp_tb_is_folio(swp_tb)) { /* * Folios are always naturally aligned in swap so * advance forward to the next boundary. Zero means no diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index bd1f74a7a5ac..bc2b1ae87a59 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1191,17 +1191,13 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma, * Check if the swap entry is cached after acquiring the src_pte * lock. Otherwise, we might miss a newly loaded swap cache folio. * - * Check swap_map directly to minimize overhead, READ_ONCE is sufficient. * We are trying to catch newly added swap cache, the only possible case is * when a folio is swapped in and out again staying in swap cache, using the * same entry before the PTE check above. The PTL is acquired and released - * twice, each time after updating the swap_map's flag. So holding - * the PTL here ensures we see the updated value. False positive is possible, - * e.g. SWP_SYNCHRONOUS_IO swapin may set the flag without touching the - * cache, or during the tiny synchronization window between swap cache and - * swap_map, but it will be gone very quickly, worst result is retry jitters. + * twice, each time after updating the swap table. So holding + * the PTL here ensures we see the updated value. */ - if (READ_ONCE(si->swap_map[swp_offset(entry)]) & SWAP_HAS_CACHE) { + if (swap_cache_has_folio(entry)) { double_pt_unlock(dst_ptl, src_ptl); return -EAGAIN; } -- 2.51.2