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Date: Tue, 24 Nov 2020 12:19:12 +0000 From: Matthew Wilcox <willy@...radead.org> To: Hugh Dickins <hughd@...gle.com> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, 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@...r.kernel.org, linux-xfs@...r.kernel.org Subject: Re: kernel BUG at fs/ext4/inode.c:LINE! On Mon, Nov 23, 2020 at 08:07:24PM -0800, Hugh Dickins wrote: > Twice now, when exercising ext4 looped on shmem huge pages, I have crashed > on the PF_ONLY_HEAD check inside PageWaiters(): ext4_finish_bio() calling > end_page_writeback() calling wake_up_page() on tail of a shmem huge page, > no longer an ext4 page at all. > > The problem is that PageWriteback is not accompanied by a page reference > (as the NOTE at the end of test_clear_page_writeback() acknowledges): as > soon as TestClearPageWriteback has been done, that page could be removed > from page cache, freed, and reused for something else by the time that > wake_up_page() is reached. > > https://lore.kernel.org/linux-mm/20200827122019.GC14765@casper.infradead.org/ > Matthew Wilcox suggested avoiding or weakening the PageWaiters() tail > check; but I'm paranoid about even looking at an unreferenced struct page, > lest its memory might itself have already been reused or hotremoved (and > wake_up_page_bit() may modify that memory with its ClearPageWaiters()). > > Then on crashing a second time, realized there's a stronger reason against > that approach. If my testing just occasionally crashes on that check, > when the page is reused for part of a compound page, wouldn't it be much > more common for the page to get reused as an order-0 page before reaching > wake_up_page()? And on rare occasions, might that reused page already be > marked PageWriteback by its new user, and already be waited upon? What > would that look like? > > It would look like BUG_ON(PageWriteback) after wait_on_page_writeback() > in write_cache_pages() (though I have never seen that crash myself). I don't think this is it. write_cache_pages() holds a reference to the page -- indeed, it holds the page lock! So this particular race cannot cause the page to get recycled. I still have no good ideas what this is :-(
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