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From:   Mina Almasry <almasrymina@google.com>
Date:   Tue, 18 Feb 2020 13:36:37 -0800
Message-ID: <CAHS8izODfKaLqWAehhR_XuN=FRgmWBE7+eCJMD2HGig8s+zvwg@mail.gmail.com>
Subject: Re: [PATCH v12 1/9] hugetlb_cgroup: Add hugetlb_cgroup reservation counter
To:     Mike Kravetz <mike.kravetz@oracle.com>
Cc:     Qian Cai <cai@lca.pw>, Andrew Morton <akpm@linux-foundation.org>,
        shuah <shuah@kernel.org>, David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Greg Thelen <gthelen@google.com>,
        open list <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        linux-kselftest@vger.kernel.org, cgroups@vger.kernel.org
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On Tue, Feb 18, 2020 at 11:25 AM Mina Almasry <almasrymina@google.com> wrote:
>
> On Tue, Feb 18, 2020 at 11:14 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
> >
> > On 2/18/20 10:35 AM, Mina Almasry wrote:
> > > On Tue, Feb 18, 2020 at 6:21 AM Qian Cai <cai@lca.pw> wrote:
> > >>
> > >> On Tue, 2020-02-11 at 15:19 -0800, Andrew Morton wrote:
> > >>> On Tue, 11 Feb 2020 13:31:20 -0800 Mina Almasry <almasrymina@google.com> wrote:
> > >>>
> > >> [ 7933.806377][T14355] ------------[ cut here ]------------
> > >> [ 7933.806541][T14355] kernel BUG at mm/hugetlb.c:490!
> > >> VM_BUG_ON(t - f <= 1);
> > >> [ 7933.806562][T14355] Oops: Exception in kernel mode, sig: 5 [#1]
> > <snip>
> > > Hi Qian,
> > >
> > > Yes this VM_BUG_ON was added by a patch in the series ("hugetlb:
> > > disable region_add file_region coalescing") so it's definitely related
> > > to the series. I'm taking a look at why this VM_BUG_ON fires. Can you
> > > confirm you reproduce this by running hugemmap06 from the ltp on a
> > > powerpc machine? Can I maybe have your config?
> > >
> > > Thanks!
> >
> > Hi Mina,
> >
> > Looking at the region_chg code again, we do a
> >
> >         resv->adds_in_progress += *out_regions_needed;
> >
> > and then potentially drop the lock to allocate the needed entries.  Could
> > anopther thread (only adding reservation for a single page) then come in
> > and notice that there are not enough entries in the cache and hit the
> > VM_BUG_ON()?
>
> Maybe. Also I'm thinking the code thinks actual_regions_needed >=
> in_regions_needed, but that doesn't seem like a guarantee. I think
> this call sequence with the same t->f range would violate that:
>
> region_chg (regions_needed=1)
> region_chg (regions_needed=1)
> region_add (fills in the range)
> region_add (in_regions_needed = 1, actual_regions_needed = 0, so
> assumptions in the code break).
>
> Luckily it seems the ltp readily reproduces this, so I'm working on
> reproducing it. I should have a fix soon, at least if I can reproduce
> it as well.

I had a bit of trouble reproducing this but I got it just now.

Makes sense I've never run into this even though others can readily
reproduce it. I happen to run my kernels on a pretty beefy 36 core
machine and in that setup things seem to execute fast and there is
never a queue of pending file_region inserts into the resv_map. Once I
limited qemu to only use 2 cores I ran into the issue right away.
Looking into a fix now.