Message-ID: <CAPQyPG6d3Sv26SiR6Xj4S5xOOy2DmrwQYO2wAwzrcg=2A0EcMQ@mail.gmail.com>
Date:	Fri, 21 Oct 2011 14:22:37 +0800
From:	Nai Xia <nai.xia@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	arekm@...-linux.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-mm@...ck.org, Mel Gorman <mgorman@...e.de>,
	jpiszcz@...idpixels.com, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pawel Sikora <pluto@...k.net>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: kernel 3.0: BUG: soft lockup: find_get_pages+0x51/0x110

On Fri, Oct 21, 2011 at 2:36 AM, Hugh Dickins <hughd@...gle.com> wrote:
> I'm travelling at the moment, my brain is not in gear, the source is not in
> front of me, and I'm not used to typing on my phone much!  Excuses, excuses
>
> I flip between thinking you are right, and I'm a fool, and thinking you are
> wrong, and I'm still a fool.

Ha, well, human brains are all weak in thoroughly searching racing state space,
while automated model checking is still far from applicable to complex
real world
like kernel source. Maybe some day someone will give out a human guided
computer aided tool to help us search the combination of all involved code paths
to valid a specific high level logic assertion.


>
> Please work it out with Linus, Andrea and Mel: I may not be able to reply
> for a couple of days - thanks.

OK.

And as a side note. Since I notice that Pawel's workload may include OOM,
I'd like to give an imaginary series of events that may trigger such an bug.

1.  do_brk() want to expand a vma, but vma_merge  failed because of
transient  ENOMEM,  but succeeded in creating a new vmas at the boundary.

    vma_a           vma_b
|----------------|---------------------|

2.  page fault in vma_b, gives it a anon_vma, then page fault in vma_a,
it reuses the anon_vma of  vma_b.


3.   vma_a remaps to somewhere irrelevant, a new vma_c is created
and linked by anon_vma_clone(). In the anon_vma chain of vma_b,
vma_c is linked after  vma_b:

    vma_a           vma_b                   vma_c
|----------------|---------------------|   |==============|

           vma_b                   vma_c
|---------------------|   |==============|



4.  vma_c remaps back to its original place where vma_a was.
Ok,  vma_merge() in copy_vma() says that this request can be merged
to vma_b, and it returns with vma_b.

5. move_page_tables moves from vma_c to vma_b,  and races with rmap_walk.
The reverse ordering of vma_b and vma_c in anon_vma chain makes
rmap_walk miss an entry in the way I explained.

Well, it seems a very tricky construction, but also seems a possible
thing to me.

Will Linus, Andrea and Mel or any other one please look into my construction
and judge if it's valid?

Thanks

Nai Xia

>
> Hugh
>
> On Oct 20, 2011 5:51 AM, "Nai Xia" <nai.xia@...il.com> wrote:
>>
>> On Thursday 20 October 2011 03:42:15 Hugh Dickins wrote:
>> > On Wed, 19 Oct 2011, Linus Torvalds wrote:
>> > > On Wed, Oct 19, 2011 at 12:43 AM, Mel Gorman <mgorman@...e.de> wrote:
>> > > >
>> > > > My vote is with the migration change. While there are occasionally
>> > > > patches to make migration go faster, I don't consider it a hot path.
>> > > > mremap may be used intensively by JVMs so I'd loathe to hurt it.
>> > >
>> > > Ok, everybody seems to like that more, and it removes code rather than
>> > > adds it, so I certainly prefer it too. Pawel, can you test that other
>> > > patch (to mm/migrate.c) that Hugh posted? Instead of the mremap vma
>> > > locking patch that you already verified for your setup?
>> > >
>> > > Hugh - that one didn't have a changelog/sign-off, so if you could
>> > > write that up, and Pawel's testing is successful, I can apply it...
>> > > Looks like we have acks from both Andrea and Mel.
>> >
>> > Yes, I'm glad to have that input from Andrea and Mel, thank you.
>> >
>> > Here we go.  I can't add a Tested-by since Pawel was reporting on the
>> > alternative patch, but perhaps you'll be able to add that in later.
>> >
>> > I may have read too much into Pawel's mail, but it sounded like he
>> > would have expected an eponymous find_get_pages() lockup by now,
>> > and was pleased that this patch appeared to have cured that.
>> >
>> > I've spent quite a while trying to explain find_get_pages() lockup by
>> > a missed migration entry, but I just don't see it: I don't expect this
>> > (or the alternative) patch to do anything to fix that problem.  I won't
>> > mind if it magically goes away, but I expect we'll need more info from
>> > the debug patch I sent Justin a couple of days ago.
>>
>> Hi Hugh,
>>
>> Will you please look into my explanation in my reply to Andrea in this
>> thread
>> and see if it's what you are seeking?
>>
>>
>> Thanks,
>>
>> Nai Xia
>>
>>
>> >
>> > Ah, I'd better send the patch separately as
>> > "[PATCH] mm: fix race between mremap and removing migration entry":
>> > Pawel's "l" makes my old alpine setup choose quoted printable when
>> > I reply to your mail.
>> >
>> > Hugh
>> >
>> > --
>> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
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>> > see: http://www.linux-mm.org/ .
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>> >
>
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