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Date: Thu, 25 Aug 2022 19:32:09 +0800 From: Zhihao Cheng <chengzhihao1@...wei.com> To: Jan Kara <jack@...e.cz> CC: <linux-ext4@...r.kernel.org>, <akpm@...ux-foundation.org>, <mgorman@...e.de>, <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>, <yukuai3@...wei.com>, <yi.zhang@...wei.com> Subject: Re: [PATCH] mm: migrate: buffer_migrate_folio_norefs() fallback migrate not uptodate pages 在 2022/8/25 18:57, Jan Kara 写道: > On Thu 25-08-22 16:01:46, Zhihao Cheng wrote: >> From: Zhang Yi <yi.zhang@...wei.com> >> >> Recently we notice that ext4 filesystem occasionally fail to read >> metadata from disk and report error message, but the disk and block >> layer looks fine. After analyse, we lockon commit 88dbcbb3a484 >> ("blkdev: avoid migration stalls for blkdev pages"). It provide a >> migration method for the bdev, we could move page that has buffers >> without extra users now, but it will lock the buffers on the page, which >> breaks a lot of current filesystem's fragile metadata read operations, >> like ll_rw_block() for common usage and ext4_read_bh_lock() for ext4, >> these helpers just trylock the buffer and skip submit IO if it lock >> failed, many callers just wait_on_buffer() and conclude IO error if the >> buffer is not uptodate after buffer unlocked. >> >> This issue could be easily reproduced by add some delay just after >> buffer_migrate_lock_buffers() in __buffer_migrate_folio() and do >> fsstress on ext4 filesystem. >> >> EXT4-fs error (device pmem1): __ext4_find_entry:1658: inode #73193: >> comm fsstress: reading directory lblock 0 >> EXT4-fs error (device pmem1): __ext4_find_entry:1658: inode #75334: >> comm fsstress: reading directory lblock 0 >> >> Something like ll_rw_block() should be used carefully and seems could >> only be safely used for the readahead case. So the best way is to fix >> the read operations in filesystem in the long run, but now let us avoid >> this issue first. This patch avoid this issue by fallback to migrate >> pages that are not uotodate like fallback_migrate_folio(), those pages >> that has buffers may probably do read operation soon. >> >> Fixes: 88dbcbb3a484 ("blkdev: avoid migration stalls for blkdev pages") >> Signed-off-by: Zhang Yi <yi.zhang@...wei.com> >> Signed-off-by: Zhihao Cheng <chengzhihao1@...wei.com> > > Thanks for the analysis and the fix! As you noted above this is actually a > bug in the filesystems that they assume that locked buffer means it is > under IO. Usually that is the case but there are other places that lock > the buffer without doing IO. Page migration is one of them, jbd2 machinery > another one, there may be others. > > So I think this really ought to be fixed in filesystems instead of papering > over the bug in the migration code. I agree this is more work but we will > reduce the technical debt, not increase it :). Honestly, ll_rw_block() > should just die. It is actively dangerous to use. Instead we should have > one call for readahead of bhs and the rest should be converted to > submit_bh() or similar calls. There are like 25 callers remaining so it > won't be even that hard. > > And then we have the same buggy code as in ll_rw_block() copied to > ext4_bread_batch() (ext4_read_bh_lock() in particular) so that needs to be > fixed as well... > > Honza You are right, Jan. I totally agree with you. It seems that I shouldn't have been lazy.
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