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From: Mina Almasry <almasrymina@google.com>
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
Subject: Re: [PATCH v12 1/9] hugetlb_cgroup: Add hugetlb_cgroup reservation counter
Date: Tue, 18 Feb 2020 13:36:37 -0800
Message-ID: <CAHS8izODfKaLqWAehhR_XuN=FRgmWBE7+eCJMD2HGig8s+zvwg@mail.gmail.com> (raw)
In-Reply-To: <CAHS8izPbMizJMNB-y9y2OViXYLStA6VT-HkWRd2hCS-5JSMwSA@mail.gmail.com>

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.


  reply index

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-11 21:31 Mina Almasry
2020-02-11 21:31 ` [PATCH v12 2/9] hugetlb_cgroup: add interface for charge/uncharge hugetlb reservations Mina Almasry
2020-02-15  0:50   ` Mike Kravetz
2020-02-16  1:21     ` David Rientjes
2020-02-11 21:31 ` [PATCH v12 3/9] hugetlb_cgroup: add reservation accounting for private mappings Mina Almasry
2020-02-11 21:31 ` [PATCH v12 4/9] hugetlb: disable region_add file_region coalescing Mina Almasry
2020-02-15  1:27   ` Mike Kravetz
2020-02-16  1:25   ` David Rientjes
2020-02-11 21:31 ` [PATCH v12 5/9] hugetlb_cgroup: add accounting for shared mappings Mina Almasry
2020-02-16  1:29   ` David Rientjes
2020-02-18 18:07   ` Mike Kravetz
2020-02-11 21:31 ` [PATCH v12 6/9] hugetlb_cgroup: support noreserve mappings Mina Almasry
2020-02-18 20:57   ` Mike Kravetz
2020-02-11 21:31 ` [PATCH v12 7/9] hugetlb: support file_region coalescing again Mina Almasry
2020-02-16  1:29   ` David Rientjes
2020-02-19  3:28   ` Mike Kravetz
2020-02-19  7:54     ` Mina Almasry
2020-02-19 23:36     ` [PATCH] hugetlb: Remove check_coalesce_bug debug code Mina Almasry
2020-02-20  0:07       ` Mike Kravetz
2020-02-11 21:31 ` [PATCH v12 8/9] hugetlb_cgroup: Add hugetlb_cgroup reservation tests Mina Almasry
2020-02-12  8:50   ` Sandipan Das
2020-02-20  0:05     ` Mina Almasry
2020-02-21  0:52   ` Mike Kravetz
2020-02-11 21:31 ` [PATCH v12 9/9] hugetlb_cgroup: Add hugetlb_cgroup reservation docs Mina Almasry
2020-02-20  0:03   ` Mina Almasry
2020-02-20  0:18   ` Mike Kravetz
2020-02-11 23:19 ` [PATCH v12 1/9] hugetlb_cgroup: Add hugetlb_cgroup reservation counter Andrew Morton
2020-02-18 14:21   ` Qian Cai
2020-02-18 18:35     ` Mina Almasry
2020-02-18 18:41       ` Qian Cai
2020-02-18 19:14       ` Mike Kravetz
2020-02-18 19:25         ` Mina Almasry
2020-02-18 21:36           ` Mina Almasry [this message]
2020-02-18 21:41             ` Mike Kravetz
2020-02-18 22:27               ` Mina Almasry
2020-02-19 19:05   ` Mina Almasry
2020-02-19 21:06     ` Andrew Morton
2020-02-20 19:22       ` Mina Almasry
2020-02-21  0:28         ` Andrew Morton
2020-02-21  0:41           ` Mike Kravetz
2020-02-21  1:52             ` Mina Almasry
2020-02-21 20:19       ` Mina Almasry

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