linux-kernel - Re: kernel BUG at mm/page-writeback.c:LINE!

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Date:   Tue, 5 Jan 2021 13:13:40 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        syzbot <syzbot+2fc0712f8f8b8b8fa0ef@...kaller.appspotmail.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: kernel BUG at mm/page-writeback.c:LINE!

On Tue, 5 Jan 2021, Linus Torvalds wrote:
> On Tue, Jan 5, 2021 at 11:31 AM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> > On Mon, Jan 4, 2021 at 7:29 PM Hugh Dickins <hughd@...gle.com> wrote:
> > >
> > > > So the one-liner of changing the "if" to "while" in
> > > > wait_on_page_writeback() should get us back to what we used to do.
> > >
> > > I think that is the realistic way to go.
> >
> > Yeah, that's what I'll do.
> 
> I took your "way to go" statement as an ack, and made it all be commit
> c2407cf7d22d ("mm: make wait_on_page_writeback() wait for multiple
> pending writebacks").

Great, thanks, I see it now.

I was going to raise a question, whether you should now revert
073861ed77b6 ("mm: fix VM_BUG_ON(PageTail) and BUG_ON(PageWriteback)"):
which would not have gone in like that if c2407cf7d22d were already in.

But if it were reverted, we'd need some other fix for the PageTail part
of it; and I still cannot think of anywhere else where we knowingly
operated on a struct page without holding a reference; and there have
been no adverse reports on its extra get_page+put_page.

So I think it's safest to leave it in.

Hugh