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Date: Tue, 24 Nov 2020 12:34:33 -0800 From: Linus Torvalds <torvalds@...ux-foundation.org> To: Matthew Wilcox <willy@...radead.org> Cc: Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>, syzbot <syzbot+3622cea378100f45d59f@...kaller.appspotmail.com>, Andreas Dilger <adilger.kernel@...ger.ca>, Ext4 Developers List <linux-ext4@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, syzkaller-bugs <syzkaller-bugs@...glegroups.com>, "Theodore Ts'o" <tytso@....edu>, Linux-MM <linux-mm@...ck.org>, Oleg Nesterov <oleg@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, "Kirill A. Shutemov" <kirill@...temov.name>, Nicholas Piggin <npiggin@...il.com>, Alex Shi <alex.shi@...ux.alibaba.com>, Qian Cai <cai@....pw>, Christoph Hellwig <hch@...radead.org>, "Darrick J. Wong" <darrick.wong@...cle.com>, William Kucharski <william.kucharski@...cle.com>, Jens Axboe <axboe@...nel.dk>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, linux-xfs <linux-xfs@...r.kernel.org> Subject: Re: kernel BUG at fs/ext4/inode.c:LINE! On Tue, Nov 24, 2020 at 12:16 PM Matthew Wilcox <willy@...radead.org> wrote: > > So my s/if/while/ suggestion is wrong and we need to do something to > prevent spurious wakeups. Unless we bury the spurious wakeup logic > inside wait_on_page_writeback() ... We can certainly make the "if()" in that loop be a "while()'. That's basically what the old code did - simply by virtue of the wakeup not happening if the writeback bit was set in wake_page_function(): if (test_bit(key->bit_nr, &key->page->flags)) return -1; of course, the race was still there - because the writeback bit might be clear at that point, but another CPU would reallocate and dirty it, and then autoremove_wake_function() would happen anyway. But back in the bad old days, the wait_on_page_bit_common() code would then double-check in a loop, so it would catch that case, re-insert itself on the wait queue, and try again. Except for the DROP case, which isn't used by writeback. Anyway, making that "if()" be a "while()" in wait_on_page_writeback() would basically re-introduce that old behavior. I don't really care, because it was the lock bit that really mattered, the writeback bit is not really all that interesting (except from a "let's fix this bug" angle) I'm not 100% sure I like the fragility of this writeback thing. Anyway, I'm certainly happy with either model, whether it be an added while() in wait_on_page_writeback(), or it be the page reference count in end_page_writeback(). Strong opinions? Linus
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