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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 24 Mar 2022 09:50:14 +0100
Message-ID: <CACT4Y+aQOziG8LGiTdyjTTHXJ4tNy2qxJ4yTPy-aUBG9CQB3_w@mail.gmail.com>
Subject: Re: [syzbot] general protection fault in list_lru_add
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        syzbot <syzbot+f8c45ccc7d5d45fc5965@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        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 Thu, 24 Mar 2022 at 09:44, Muchun Song <songmuchun@bytedance.com> wrote:
>
> On Thu, Mar 24, 2022 at 11:05 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > On Wed, Mar 23, 2022 at 7:19 PM Muchun Song <songmuchun@bytedance.com> wrote:
> > >
> > > After this commit, the rules of dentry allocations changed.
> > > The dentry should be allocated by kmem_cache_alloc_lru()
> >
> > Yeah, I looked at that, but I can't find any way there could be other
> > allocations - not only are there strict rules how to initialize
> > everything, but the dentries are free'd using
> >
> >         kmem_cache_free(dentry_cache, dentry);
> >
> > and as a result if they were allocated any other way I would expect
> > things would go south very quickly.
> >
> > The only other thing I could come up with is some breakage in the
> > superblock lifetime so that &dentry->d_sb->s_dentry_lru would have
> > problems, but again, this is *such* core code and not some unusual
> > path, that I would be very very surprised if it wouldn't have
> > triggered other issues long long ago.
> >
> > That's why I'd be more inclined to worry about the list_lru code being
> > somehow broken.
> >
>
> I also have the same concern.  I have been trying for a few hours to
> reproduce this issue, but it didn't oops on my test machine.  And I'll
> continue reproducing this.

syzbot triggered it 222 times in a day, so it's most likely real:
https://syzkaller.appspot.com/bug?extid=f8c45ccc7d5d45fc5965

There are 2 reproducers, but they look completely different. May be a race.
You may also try to use syzbot's patch testing feature to get some
additional debug info.