Re: + mm-hugetlb-fix-race-when-migrate-pages.patch added to -mm tree

From: Michal Hocko <mhocko@kernel.org>
To: zhong jiang <zhongjiang@huawei.com>,
	Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: "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>
Subject: Re: + mm-hugetlb-fix-race-when-migrate-pages.patch added to -mm tree
Date: Tue, 26 Jul 2016 09:58:48 +0200	[thread overview]
Message-ID: <20160726075847.GG32462@dhcp22.suse.cz> (raw)
In-Reply-To: <20160722071737.GA3785@hori1.linux.bs1.fc.nec.co.jp>

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.

> 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.

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?

-- 
Michal Hocko
SUSE Labs

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