Messages in this thread | | | Date | Wed, 04 Feb 2004 18:54:54 -0800 | From | Janet Morgan <> | Subject | Re: [PATCH 2.6.2-rc3-mm1] DIO read race fix |
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Andrew Morton wrote:
>Daniel McNeil <daniel@osdl.org> wrote: > > >>I have found (finally) the problem causing DIO reads racing with >>buffered writes to see uninitialized data on ext3 file systems >>(which is what I have been testing on). >> >> > >What kernel? If -mm, is this the only remaining buffered-vs-direct >problem? > > > I think there was consensus on two other patches along the way:
http://marc.theaimsgroup.com/?l=linux-kernel&m=107286971311559&w=2 http://marc.theaimsgroup.com/?l=linux-aio&m=107291089712224&w=2
-Janet
>>The problem is caused by the changes to __block_write_page_full() >>and a race with journaling: >> >>journal_commit_transaction() -> ll_rw_block() -> submit_bh() >> >>ll_rw_block() locks the buffer, clears buffer dirty and calls >>submit_bh() >> >>A racing __block_write_full_page() (from ext3_ordered_writepage()) >> >> would see that buffer_dirty() is not set because the i/o >> is still in flight, so it would not do a bh_submit() >> >> It would SetPageWriteback() and unlock_page() and then >> see that no i/o was submitted and call end_page_writeback() >> (with the i/o still in flight). >> >>This would allow the DIO code to issue the DIO read while buffer writes >>are still in flight. The i/o can be reordered by i/o scheduling and >>the DIO can complete BEFORE the writebacks complete. Thus the DIO >>sees the old uninitialized data. >> >> > >ow. How'd you work this out? > > > >>Here is a quick hack that fixes it, but I am not sure if this the >>proper long term fix. >> >> > >The problem is that ext3 and the VFS are using different paradigms. VFS is >all page-based, but ext3 is all block-based. One or the other needs to do >something nasty. > >One approach would be to change the JBD write_out_data_locked loop to use >block_write_full_page(bh->b_page) instead of buffer_head operations. But >that could get hairy with blocksize < PAGE_SIZE. > >Thanks for working this out. Let me ponder it for a bit. > > >
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