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Tue, 12 Dec 2023 13:37:13 +0000 (UTC) Received: from dovecot-director2.suse.de ([10.150.64.162]) by imap2.dmz-prg2.suse.org with ESMTPSA id ODP2IglieGXcQwAAn2gu4w (envelope-from ); Tue, 12 Dec 2023 13:37:13 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 3C652A06E5; Tue, 12 Dec 2023 14:37:13 +0100 (CET) Date: Tue, 12 Dec 2023 14:37:13 +0100 From: Jan Kara To: Baokun Li Cc: Jan Kara , linux-mm@kvack.org, linux-ext4@vger.kernel.org, tytso@mit.edu, adilger.kernel@dilger.ca, 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] mm/filemap: avoid buffered read/write race to read inconsistent data Message-ID: <20231212133713.bihojdsnccmadcpg@quack3> References: <20231212093634.2464108-1-libaokun1@huawei.com> <20231212124157.ew6q6jp2wsezvqzd@quack3> <9fdebd0a-ac10-e193-b245-7678fa708c82@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <9fdebd0a-ac10-e193-b245-7678fa708c82@huawei.com> X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: B8FB040003 X-Stat-Signature: ehzihfx89dnzysdagah66k5f9ao6hayx X-Rspam-User: X-HE-Tag: 1702388235-741993 X-HE-Meta: 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 /tmef6FA X4jgs6MwsRFSH+99nzbIws5wvnScFYMYHu6dVTkW+agjDXjVhdPNHCHveLa0szx88/73D9BQ7E8hhf2nF9xF3B6+9kcJEpq8M2MBURH6jv7/ES/oehAL3y2GS9LA9VPWBFWHVYG8KHmxLnjy/h6qSCyvmputpNBEg5WMIMdhe1RPnNkBQAA+JBYj07szoDuiUW/IC/j4w49Y+X16kcmiGdv8cplvRerWRCzZbHMh5JkhtefjizOsb2dWe4z0IUmKt6shMWJH99v1vUyZSo/G88+CxJ6yMAtc94YZb2LLMzuZ8pR+mmPLOcpnGTOJqss/mxf09aQkNbwXAvl0r5YXBp/EQr8Z+i/eQt9VVFbhmUYzCmLN7wlSGyVY7nes2EcgCIYb+ 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 Tue 12-12-23 21:16:16, Baokun Li wrote: > On 2023/12/12 20:41, Jan Kara wrote: > > On Tue 12-12-23 17:36:34, 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 > > AFAIK x86 can also reorder loads vs loads so the problem can in theory > > happen on x86 as well. > > According to what I read in the /perfbook /at the link below, > >  Loads Reordered After Loads does not happen on x86. > > pdf sheet 562 corresponds to page 550, > >    Table 15.5: Summary of Memory Ordering > > https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook-1c.2023.06.11a.pdf Indeed. I stand corrected! Thanks for the link. Honza -- Jan Kara SUSE Labs, CR