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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 5 Jan 2021 13:22:49 -0800
Message-ID: <CAHk-=wiGHdRRmZtrcUt6g6hmATkmueNVLNnSHSBoUp-v-Qy5sw@mail.gmail.com>
Subject: Re: kernel BUG at mm/page-writeback.c:LINE!
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        syzbot <syzbot+2fc0712f8f8b8b8fa0ef@syzkaller.appspotmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
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On Tue, Jan 5, 2021 at 1:13 PM Hugh Dickins <hughd@google.com> wrote:
>
> 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.

Honestly, even if it wasn't for that PageTail issue, I think
073861ed77b6 is just the right thing to do anyway. It just feels _so_
much safer to not have the possibility of that page wait thing
following while the page is possibly then being free'd and re-used at
the same time.

So I think the only reason to revert that commit would be if we were
to find that it's a huge performance problem to raise the page
refcount temporarily. Which I think is very unlikely (since we already
dirty the page structure due to the page flags modification - although
they are far enough apart that it might be a different cacheline).

                   Linus