linux-kernel - Re: [syzbot] [mm?] kernel BUG in vma_replace

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Message-ID: <CAJuCfpEAZbtUrwrfqd+PWJv9efVy1HRbqrLUYAP4rYRvk0vWug@mail.gmail.com>
Date:   Thu, 14 Sep 2023 20:00:45 +0000
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     syzbot <syzbot+b591856e0f0139f83023@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] kernel BUG in vma_replace_policy

On Thu, Sep 14, 2023 at 7:09 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Sep 14, 2023 at 06:20:56PM +0000, Suren Baghdasaryan wrote:
> > I think I found the problem and the explanation is much simpler. While
> > walking the page range, queue_folios_pte_range() encounters an
> > unmovable page and queue_folios_pte_range() returns 1. That causes a
> > break from the loop inside walk_page_range() and no more VMAs get
> > locked. After that the loop calling mbind_range() walks over all VMAs,
> > even the ones which were skipped by queue_folios_pte_range() and that
> > causes this BUG assertion.
> >
> > Thinking what's the right way to handle this situation (what's the
> > expected behavior here)...
> > I think the safest way would be to modify walk_page_range() and make
> > it continue calling process_vma_walk_lock() for all VMAs in the range
> > even when __walk_page_range() returns a positive err. Any objection or
> > alternative suggestions?
>
> So we only return 1 here if MPOL_MF_MOVE* & MPOL_MF_STRICT were
> specified.  That means we're going to return an error, no matter what,
> and there's no point in calling mbind_range().  Right?
>
> +++ b/mm/mempolicy.c
> @@ -1334,6 +1334,8 @@ static long do_mbind(unsigned long start, unsigned long len,
>         ret = queue_pages_range(mm, start, end, nmask,
>                           flags | MPOL_MF_INVERT, &pagelist, true);
>
> +       if (ret == 1)
> +               ret = -EIO;
>         if (ret < 0) {
>                 err = ret;
>                 goto up_out;
>
> (I don't really understand this code, so it can't be this simple, can
> it?  Why don't we just return -EIO from queue_folios_pte_range() if
> this is the right answer?)

Yeah, I'm trying to understand the expected behavior of this function
to make sure we are not missing anything. I tried a simple fix that I
suggested in my previous email and it works but I want to understand a
bit more about this function's logic before posting the fix.