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Date: Wed, 27 Jul 2016 16:44:05 +0200
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: + mm-hugetlb-fix-race-when-migrate-pages.patch added to -mm tree
Message-ID: <20160727144400.GB21859@dhcp22.suse.cz>
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To: zhong jiang <zhongjiang@huawei.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>, "akpm@linux-foundation.org" <akpm@linux-foundation.org>, "qiuxishi@huawei.com" <qiuxishi@huawei.com>, "vbabka@suse.cz" <vbabka@suse.cz>, "mm-commits@vger.kernel.org" <mm-commits@vger.kernel.org>, Mike Kravetz <mike.kravetz@oracle.com>, Mel Gorman <mgorman@suse.de>, "linux-mm@kvack.org" <linux-mm@kvack.org>

On Tue 26-07-16 22:04:16, zhong jiang wrote:
> On 2016/7/26 15:58, Michal Hocko wrote:
> > On Fri 22-07-16 07:17:37, Naoya Horiguchi wrote:
> > [...]
> >> I think that (src_pte != dst_pte) can happen and that's ok if there's no
> >> migration entry. 
> > We have discussed that with Naoya off-list and couldn't find a scenario
> > when parent would have !shared pmd while child would have it. The only
> > plausible scenario was that parent created and poppulated mapping smaller
> > than 1G and then enlarged it later on so the child would see sharedable
> > pud. This doesn't seem to be possible because vma_merge would bail out
> > due to VM_SPECIAL check.

> I do not understand that the process must have vm_special flags. if
> vm_special enable, the process must not be expanded. and what does it
> matter about vma_merge ??

See 
	if (vm_flags & VM_SPECIAL)
		return NULL;

in vma_merge.

> >> But even if we have both of normal entry and migration entry
> >> for one hugepage, that still looks fine to me because the running migration
> >> operation fails (because there remains mapcounts on the source hugepage),
> >> and all migration entries are turned back to normal entries pointing to the
> >> source hugepage.
>
> In one case, try_to_unmap_one is first exec and successfully, mapcount
> turn into zero. then we get the pte lock, if src_pte!-dst_pte, it
> maybe lead to the dst_pte is from migrate pte to normal pte, while the
> normal pte turn into migaret pte,, is right ?

I am sorry but I have hard time following your arguments here. Could you
be more specific please?

> > Agreed.
> >
> >> Could you try to see and share what happens on your workload with
> >> Michal's patch?
> >
> > Zhong Jiang did you have chance to retest with the BUG_ON changed?

Anything for this?
-- 
Michal Hocko
SUSE Labs

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