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From:   Mina Almasry <almasrymina@google.com>
Date:   Tue, 18 Feb 2020 14:27:57 -0800
Message-ID: <CAHS8izO0LikomAQEbGacuBfcczZXJzYWjXX1bu=hrF5ZBCT5Ng@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 1:41 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 2/18/20 1:36 PM, Mina Almasry wrote:
> > 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.
>
> This may not be optimal, but it resolves the issue for me.  I just put it
> together to test the theory that the region_chg code was at fault.

Thanks! Just sent out a similar patch "[PATCH -next] mm/hugetlb: Fix
file_region entry allocations"