Re: [syzbot] [mm?] kernel BUG in vma_replace_policy

[Matthew Wilcox <willy@infradead.org>]

On Thu, Sep 14, 2023 at 08:53:59PM +0000, Suren Baghdasaryan wrote:
> On Thu, Sep 14, 2023 at 8:00 PM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > On Thu, Sep 14, 2023 at 7:09 PM Matthew Wilcox <willy@infradead.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.
> 
> So, current functionality is that after queue_pages_range() encounters
> an unmovable page, terminates the loop and returns 1, mbind_range()
> will still be called for the whole range
> (https://elixir.bootlin.com/linux/latest/source/mm/mempolicy.c#L1345),
> all pages in the pagelist will be migrated
> (https://elixir.bootlin.com/linux/latest/source/mm/mempolicy.c#L1355)
> and only after that the -EIO code will be returned
> (https://elixir.bootlin.com/linux/latest/source/mm/mempolicy.c#L1362).
> So, if we follow Matthew's suggestion we will be altering the current
> behavior which I assume is not what we want to do.

Right, I'm intentionally changing the behaviour.  My thinking is
that mbind(MPOL_MF_MOVE | MPOL_MF_STRICT) is going to fail.  Should
such a failure actually move the movable pages before reporting that
it failed?  I don't know.

> The simple fix I was thinking about that would not alter this behavior
> is smth like this:

I don't like it, but can we run it past syzbot to be sure it solves the
issue and we're not chasing a ghost here?