linux-kernel - Re: [syzbot] memory leak in xas

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Date:   Tue, 12 Jul 2022 08:54:28 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        syzbot <syzbot+a785d07959bc94837d51@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com,
        "Zach O'Keefe" <zokeefe@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Liam Howlett <liam.howlett@...cle.com>
Subject: Re: [syzbot] memory leak in xas_create

On Mon, 11 Jul 2022 at 22:47, Matthew Wilcox <willy@...radead.org> wrote:
>
> On Mon, Jul 11, 2022 at 01:38:08PM -0700, Andrew Morton wrote:
> > On Sat, 09 Jul 2022 00:13:23 -0700 syzbot <syzbot+a785d07959bc94837d51@...kaller.appspotmail.com> wrote:
> >
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    c1084b6c5620 Merge tag 'soc-fixes-5.19-2' of git://git.ker..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=14967ccc080000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=916233b7694a38ff
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=a785d07959bc94837d51
> > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=122ae834080000
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+a785d07959bc94837d51@...kaller.appspotmail.com
> > >
> > > 2022/07/05 05:22:17 executed programs: 828
> > > 2022/07/05 05:22:23 executed programs: 846
> > > 2022/07/05 05:22:30 executed programs: 866
> > > 2022/07/05 05:22:37 executed programs: 875
> > > BUG: memory leak
> >
> > Thanks.  Presumably due to khugepaged changes.
>
> Huh, I was expecting it to be something I'd messed up.  I've been
> looking at it today, but no luck figuring it out so far.
>
> > Can we expect a bisection search?
>
> We only have a syz reproducer so far, and if I understand correctly,
> it's probably because this is a flaky test (because it's trying to
> find something that's a race condition).
>
> I expect a bisection search to go badly wrong if this is true.

Is it possible that parts of xas are not freed on the error paths?
I don't immediately see where anything is freed on these error paths:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/xarray.c?id=c1084b6c5620a743f86947caca66d90f24060f56#n681
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/xarray.c?id=c1084b6c5620a743f86947caca66d90f24060f56#n721
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/khugepaged.c?id=c1084b6c5620a743f86947caca66d90f24060f56#n1675