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Wed, 18 Mar 2026 15:30:02 -0700 (PDT) From: Nhat Pham To: kasong@tencent.com Cc: Liam.Howlett@oracle.com, akpm@linux-foundation.org, apopple@nvidia.com, axelrasmussen@google.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, bhe@redhat.com, byungchul@sk.com, cgroups@vger.kernel.org, chengming.zhou@linux.dev, chrisl@kernel.org, corbet@lwn.net, david@kernel.org, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jannh@google.com, joshua.hahnjy@gmail.com, lance.yang@linux.dev, lenb@kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-pm@vger.kernel.org, lorenzo.stoakes@oracle.com, matthew.brost@intel.com, mhocko@suse.com, muchun.song@linux.dev, npache@redhat.com, nphamcs@gmail.com, pavel@kernel.org, peterx@redhat.com, peterz@infradead.org, pfalcato@suse.de, rafael@kernel.org, rakie.kim@sk.com, roman.gushchin@linux.dev, rppt@kernel.org, ryan.roberts@arm.com, shakeel.butt@linux.dev, shikemeng@huaweicloud.com, surenb@google.com, tglx@kernel.org, vbabka@suse.cz, weixugc@google.com, ying.huang@linux.alibaba.com, yosry.ahmed@linux.dev, yuanchu@google.com, zhengqi.arch@bytedance.com, ziy@nvidia.com, kernel-team@meta.com, riel@surriel.com Subject: [PATCH v4 05/21] mm/swap: add a new function to check if a swap entry is in swap cached. Date: Wed, 18 Mar 2026 15:29:36 -0700 Message-ID: <20260318222953.441758-6-nphamcs@gmail.com> X-Mailer: git-send-email 2.52.0 In-Reply-To: <20260318222953.441758-1-nphamcs@gmail.com> References: <20260318222953.441758-1-nphamcs@gmail.com> Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Userfaultfd checks whether a swap entry is in swapcache. This is currently done by directly looking at the swapfile's swap map - however, the swap cached state will soon be managed at the virtual swap layer. Abstract away this function. Signed-off-by: Nhat Pham --- include/linux/swap.h | 6 ++++++ mm/swapfile.c | 15 +++++++++++++++ mm/userfaultfd.c | 3 +-- 3 files changed, 22 insertions(+), 2 deletions(-) diff --git a/include/linux/swap.h b/include/linux/swap.h index 3da637b218baf..f91a442ac0e82 100644 --- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -435,6 +435,7 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr); int __swap_count(swp_entry_t entry); bool swap_entry_swapped(struct swap_info_struct *si, swp_entry_t entry); int swp_swapcount(swp_entry_t entry); +bool is_swap_cached(swp_entry_t entry); /* Swap cache API (mm/swap_state.c) */ static inline unsigned long total_swapcache_pages(void) @@ -554,6 +555,11 @@ static inline int swp_swapcount(swp_entry_t entry) return 0; } +static inline bool is_swap_cached(swp_entry_t entry) +{ + return false; +} + static inline int folio_alloc_swap(struct folio *folio) { return -EINVAL; diff --git a/mm/swapfile.c b/mm/swapfile.c index 46da28c533bbe..0471a965f222b 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -194,6 +194,21 @@ static bool swap_only_has_cache(struct swap_info_struct *si, return true; } +/** + * is_swap_cached - check if the swap entry is cached + * @entry: swap entry to check + * + * Check swap_map directly to minimize overhead, READ_ONCE is sufficient. + * + * Returns true if the swap entry is cached, false otherwise. + */ +bool is_swap_cached(swp_entry_t entry) +{ + struct swap_info_struct *si = __swap_entry_to_info(entry); + + return READ_ONCE(si->swap_map[swp_offset(entry)]) & SWAP_HAS_CACHE; +} + static bool swap_is_last_map(struct swap_info_struct *si, unsigned long offset, int nr_pages, bool *has_cache) { diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 25f89eba0438c..98be764fb3ecd 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1190,7 +1190,6 @@ 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 @@ -1200,7 +1199,7 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma, * 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. */ - if (READ_ONCE(si->swap_map[swp_offset(entry)]) & SWAP_HAS_CACHE) { + if (is_swap_cached(entry)) { double_pt_unlock(dst_ptl, src_ptl); return -EAGAIN; } -- 2.52.0