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[66.187.233.206]) by smtp.gmail.com with ESMTPSA id r1sm442348qtu.83.2020.02.10.09.00.42 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 10 Feb 2020 09:00:43 -0800 (PST) From: Qian Cai To: akpm@linux-foundation.org Cc: elver@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Qian Cai Subject: [PATCH -next] mm/filemap: fix a data race in filemap_fault() Date: Mon, 10 Feb 2020 12:00:29 -0500 Message-Id: <1581354029-20154-1-git-send-email-cai@lca.pw> X-Mailer: git-send-email 1.8.3.1 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: struct file_ra_state ra.mmap_miss could be accessed concurrently during page faults as noticed by KCSAN, BUG: KCSAN: data-race in filemap_fault / filemap_map_pages write to 0xffff9b1700a2c1b4 of 4 bytes by task 3292 on cpu 30: filemap_fault+0x920/0xfc0 do_sync_mmap_readahead at mm/filemap.c:2384 (inlined by) filemap_fault at mm/filemap.c:2486 __xfs_filemap_fault+0x112/0x3e0 [xfs] xfs_filemap_fault+0x74/0x90 [xfs] __do_fault+0x9e/0x220 do_fault+0x4a0/0x920 __handle_mm_fault+0xc69/0xd00 handle_mm_fault+0xfc/0x2f0 do_page_fault+0x263/0x6f9 page_fault+0x34/0x40 read to 0xffff9b1700a2c1b4 of 4 bytes by task 3313 on cpu 32: filemap_map_pages+0xc2e/0xd80 filemap_map_pages at mm/filemap.c:2625 do_fault+0x3da/0x920 __handle_mm_fault+0xc69/0xd00 handle_mm_fault+0xfc/0x2f0 do_page_fault+0x263/0x6f9 page_fault+0x34/0x40 Reported by Kernel Concurrency Sanitizer on: CPU: 32 PID: 3313 Comm: systemd-udevd Tainted: G W L 5.5.0-next-20200210+ #1 Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019 ra.mmap_miss is used to contribute the readahead decisions, a data race could be undesirable. Since the stores are aligned and less than word-size, assume they are safe. Thus, fixing it by adding READ_ONCE() for the loads except those places comparing to zero where they are safe. Signed-off-by: Qian Cai --- mm/filemap.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/mm/filemap.c b/mm/filemap.c index 1784478270e1..b6c1d37f7ea3 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2380,14 +2380,14 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf) } /* Avoid banging the cache line if not needed */ - if (ra->mmap_miss < MMAP_LOTSAMISS * 10) + if (READ_ONCE(ra->mmap_miss) < MMAP_LOTSAMISS * 10) ra->mmap_miss++; /* * Do we miss much more than hit in this file? If so, * stop bothering with read-ahead. It will only hurt. */ - if (ra->mmap_miss > MMAP_LOTSAMISS) + if (READ_ONCE(ra->mmap_miss) > MMAP_LOTSAMISS) return fpin; /* @@ -2418,7 +2418,7 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf, /* If we don't want any read-ahead, don't bother */ if (vmf->vma->vm_flags & VM_RAND_READ) return fpin; - if (ra->mmap_miss > 0) + if (data_race(ra->mmap_miss > 0)) ra->mmap_miss--; if (PageReadahead(page)) { fpin = maybe_unlock_mmap_for_io(vmf, fpin); @@ -2622,7 +2622,7 @@ void filemap_map_pages(struct vm_fault *vmf, if (page->index >= max_idx) goto unlock; - if (file->f_ra.mmap_miss > 0) + if (data_race(file->f_ra.mmap_miss > 0)) file->f_ra.mmap_miss--; vmf->address += (xas.xa_index - last_pgoff) << PAGE_SHIFT;