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Wed, 13 Dec 2023 09:31:32 +0000 (UTC) Received: from dovecot-director2.suse.de ([10.150.64.162]) by imap2.dmz-prg2.suse.org with ESMTPSA id vjNmAPR5eWVFYgAAn2gu4w (envelope-from ); Wed, 13 Dec 2023 09:31:32 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 9CAE7A07E0; Wed, 13 Dec 2023 10:31:31 +0100 (CET) Date: Wed, 13 Dec 2023 10:31:31 +0100 From: Jan Kara To: Baokun Li Cc: linux-mm@kvack.org, linux-ext4@vger.kernel.org, tytso@mit.edu, adilger.kernel@dilger.ca, jack@suse.cz, willy@infradead.org, akpm@linux-foundation.org, david@fromorbit.com, hch@infradead.org, ritesh.list@gmail.com, linux-kernel@vger.kernel.org, yi.zhang@huawei.com, yangerkun@huawei.com, yukuai3@huawei.com, stable@kernel.org Subject: Re: [RFC PATCH v2] mm/filemap: avoid buffered read/write race to read inconsistent data Message-ID: <20231213093131.zk6rrpdfiwf263as@quack3> References: <20231213062324.739009-1-libaokun1@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20231213062324.739009-1-libaokun1@huawei.com> X-Spamd-Bar: +++++++++ X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 4B6BBA0003 X-Stat-Signature: 3xa5s4hurw5fxk8za39j755rqmbmtj4x X-Rspam-User: X-HE-Tag: 1702459895-557099 X-HE-Meta: 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 tZxVsHaf Ato2y7HaDDYC5AiGTMvUGqlp/8WLWW08zyW430kCK+8LVNMtbnuTmG+KwWNOMvCgm9wRL6P3q7eqCjpw4P/W9TYvJfOG76eB3sJOQkDBNRja8gmoqIaQYblFvqdb9E1ARlXl2xMrb/3talPT+CEZyYsrHYa/+imk/pUAGP4t7jWMDFX4tN4Ven6EocBETN3bnJoK5bejosdDFSyfq8EtUKsfD7bK3nMgqFh0NH19Z/uAZgLLcWTZB79XscbWQYSR8ERNMhh0d4w2mJn6XFEW3h3cLbVW6GY2/qXyu5Lnb618G9ug44+M0gh1ug3hEPvgalS3mrAkr0UIZ5jf6al5kJWzvW9r2dKazsm9BZ4D2OLg0yN7Fh4VuQudMA8JE+mANSHae8fmM+f98vKRb30Sv64JaimKDLYuCcgpp4oht/54cnXR9zmSr+2U+Vw== 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: On Wed 13-12-23 14:23:24, Baokun Li wrote: > The following concurrency may cause the data read to be inconsistent with > the data on disk: > > cpu1 cpu2 > ------------------------------|------------------------------ > // Buffered write 2048 from 0 > ext4_buffered_write_iter > generic_perform_write > copy_page_from_iter_atomic > ext4_da_write_end > ext4_da_do_write_end > block_write_end > __block_commit_write > folio_mark_uptodate > // Buffered read 4096 from 0 smp_wmb() > ext4_file_read_iter set_bit(PG_uptodate, folio_flags) > generic_file_read_iter i_size_write // 2048 > filemap_read unlock_page(page) > filemap_get_pages > filemap_get_read_batch > folio_test_uptodate(folio) > ret = test_bit(PG_uptodate, folio_flags) > if (ret) > smp_rmb(); > // Ensure that the data in page 0-2048 is up-to-date. > > // New buffered write 2048 from 2048 > ext4_buffered_write_iter > generic_perform_write > copy_page_from_iter_atomic > ext4_da_write_end > ext4_da_do_write_end > block_write_end > __block_commit_write > folio_mark_uptodate > smp_wmb() > set_bit(PG_uptodate, folio_flags) > i_size_write // 4096 > unlock_page(page) > > isize = i_size_read(inode) // 4096 > // Read the latest isize 4096, but without smp_rmb(), there may be > // Load-Load disorder resulting in the data in the 2048-4096 range > // in the page is not up-to-date. > copy_page_to_iter > // copyout 4096 > > In the concurrency above, we read the updated i_size, but there is no read > barrier to ensure that the data in the page is the same as the i_size at > this point, so we may copy the unsynchronized page out. Hence adding the > missing read memory barrier to fix this. > > This is a Load-Load reordering issue, which only occurs on some weak > mem-ordering architectures (e.g. ARM64, ALPHA), but not on strong > mem-ordering architectures (e.g. X86). And theoretically the problem > doesn't only happen on ext4, filesystems that call filemap_read() but > don't hold inode lock (e.g. btrfs, f2fs, ubifs ...) will have this > problem, while filesystems with inode lock (e.g. xfs, nfs) won't have > this problem. > > Cc: stable@kernel.org > Signed-off-by: Baokun Li Thanks for the fix. It looks good to me. Feel free to add: Reviewed-by: Jan Kara Honza > --- > V1->V2: > Change the comment to the one suggested by Jan Kara. > > mm/filemap.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/mm/filemap.c b/mm/filemap.c > index 71f00539ac00..10c4583c06ce 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -2607,6 +2607,15 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter, > goto put_folios; > end_offset = min_t(loff_t, isize, iocb->ki_pos + iter->count); > > + /* > + * Pairs with a barrier in > + * block_write_end()->mark_buffer_dirty() or other page > + * dirtying routines like iomap_write_end() to ensure > + * changes to page contents are visible before we see > + * increased inode size. > + */ > + smp_rmb(); > + > /* > * Once we start copying data, we don't want to be touching any > * cachelines that might be contended: > -- > 2.31.1 > -- Jan Kara SUSE Labs, CR