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Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kairui Song , Qi Zheng , Shakeel Butt , Axel Rasmussen , Yuanchu Xie , Wei Xu , Baoquan He , "Paul E . McKenney" , Pedro Falcato , Harry Yoo , Luis Chamberlain , Ilya Gladyshev , Jaegeuk Kim , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, fuse-devel@lists.linux.dev, kernel-team@meta.com Subject: [RFC PATCH 17/18] mm/shmem: read without folio references Date: Mon, 19 Oct 2026 16:49:10 +0100 Message-ID: <20261019154913.1765974-18-kas@kernel.org> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20261019154913.1765974-1-kas@kernel.org> References: <20261019154913.1765974-1-kas@kernel.org> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit shmem_file_read_iter() takes a reference on every folio it reads, so small reads of one tmpfs file from many CPUs bounce the folio's refcount. Copy with filemap_read_rcu() first, and continue with the existing loop where it stops. Holes, swap entries and fallocated folios that are not uptodate all end the copy. So does a hwpoisoned page, which shmem keeps in the page cache and reports with -EIO. This drops the folio lock as well as the reference. shmem_get_folio() held it only for the lookup, and the copy ran after folio_unlock(), so a read was never atomic against a write or a hole punch. Now a read also does not wait for one that holds the lock, as filemap_read() does not. shmem does no readahead, and on its folios the PG_readahead bit is PG_reclaim, which drop_caches can leave set on dirty tmpfs folios. Do not let it end the copy there. Assisted-by: LLM Signed-off-by: Kiryl Shutsemau (Meta) --- mm/filemap.c | 7 ++++--- mm/internal.h | 2 ++ mm/shmem.c | 11 +++++++++-- 3 files changed, 15 insertions(+), 5 deletions(-) diff --git a/mm/filemap.c b/mm/filemap.c index 4b5b7ba78f39..cde7bc68b32d 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2837,9 +2837,10 @@ static void filemap_end_dropbehind_read(struct folio *folio) static bool filemap_read_rcu_can_copy(struct folio *folio, pgoff_t index, pgoff_t last_index) { - if (!folio_test_uptodate(folio)) + if (!folio_test_uptodate(folio) || folio_test_dropbehind(folio)) return false; - if (folio_test_readahead(folio) || folio_test_dropbehind(folio)) + /* shmem does no readahead: there the bit is PG_reclaim */ + if (folio_test_readahead(folio) && !folio_test_swapbacked(folio)) return false; /* memory_failure() waits only for readers that got past this */ if (folio_contain_hwpoisoned_page(folio)) @@ -3017,7 +3018,7 @@ static bool filemap_read_rcu_batch(struct kiocb *iocb, struct iov_iter *iter, * Copy from the page cache without folio references, in batches, and * return how much was copied. The slow path reads the rest. */ -static size_t filemap_read_rcu(struct kiocb *iocb, struct iov_iter *iter, +size_t filemap_read_rcu(struct kiocb *iocb, struct iov_iter *iter, loff_t last_pos) { /* The page the previous read ended in; ULONG_MAX if none */ diff --git a/mm/internal.h b/mm/internal.h index 18391dc50418..ae9a4e511c82 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -1249,6 +1249,8 @@ struct migration_target_control { size_t splice_folio_into_pipe(struct pipe_inode_info *pipe, struct folio *folio, loff_t fpos, size_t size); void filemap_read_accessed(struct folio *folio, loff_t pos, loff_t last_pos); +size_t filemap_read_rcu(struct kiocb *iocb, struct iov_iter *iter, + loff_t last_pos); static inline bool vma_is_single_threaded_private(struct vm_area_struct *vma) { diff --git a/mm/shmem.c b/mm/shmem.c index 43ddb50974a5..e6c18e745f81 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -3313,7 +3313,14 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to) pgoff_t index; unsigned long offset; int error = 0; - ssize_t retval = 0; + ssize_t retval; + + retval = filemap_read_rcu(iocb, to, last_pos); + if (retval) { + last_pos = iocb->ki_pos; + if (!iov_iter_count(to)) + goto out; + } for (;;) { struct folio *folio = NULL; @@ -3417,7 +3424,7 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to) } cond_resched(); } - +out: file_accessed(file); /* Readers sharing the file would bounce the line for nothing */ if (file->f_ra.prev_pos != last_pos) -- 2.54.0