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

From: Michal Hocko <mhocko@kernel.org>
To: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"zhongjiang@huawei.com" <zhongjiang@huawei.com>,
	"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: Thu, 21 Jul 2016 12:29:11 +0200	[thread overview]
Message-ID: <20160721102911.GF26379@dhcp22.suse.cz> (raw)
In-Reply-To: <20160721081355.GB25398@hori1.linux.bs1.fc.nec.co.jp>

On Thu 21-07-16 08:13:55, Naoya Horiguchi wrote:
> On Thu, Jul 21, 2016 at 09:43:40AM +0200, Michal Hocko wrote:
> > We have further discussed the patch and I believe it is not correct. See [1].
> > I am proposing the following alternative.
> >
> > [1] http://lkml.kernel.org/r/20160720132431.GM11249@dhcp22.suse.cz
> > ---
> > From b1e9b3214f1859fdf7d134cdcb56f5871933539c Mon Sep 17 00:00:00 2001
> > From: Michal Hocko <mhocko@suse.com>
> > Date: Thu, 21 Jul 2016 09:28:13 +0200
> > Subject: [PATCH] mm, hugetlb: fix huge_pte_alloc BUG_ON
> >
> > Zhong Jiang has reported a BUG_ON from huge_pte_alloc hitting when he
> > runs his database load with memory online and offline running in
> > parallel. The reason is that huge_pmd_share might detect a shared pmd
> > which is currently migrated and so it has migration pte which is
> > !pte_huge.
> >
> > There doesn't seem to be any easy way to prevent from the race and in
> > fact seeing the migration swap entry is not harmful. Both callers of
> > huge_pte_alloc are prepared to handle them. copy_hugetlb_page_range
> > will copy the swap entry and make it COW if needed. hugetlb_fault will
> > back off and so the page fault is retries if the page is still under
> > migration and waits for its completion in hugetlb_fault.
> >
> > That means that the BUG_ON is wrong and we should update it. Let's
> > simply check that all present ptes are pte_huge instead.
> >
> > Reported-by: zhongjiang <zhongjiang@huawei.com>
> > Signed-off-by: Michal Hocko <mhocko@suse.com>
> 
> In the early days of hugetlb, we had an assumption that !pte_none is
> equivalent to pmd_present() because there was no valid non-present entry
> on huge_pte. Situation has changed by hugepage migration and/or hwpoison,
> so we have to care about the separation of these two, and make sure that
> pte_present is true before checking pte_huge.
> 
> So I think this change is right. Thank you Zhong, Michal.
> 
> Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Thank you for double checking Naoya!

IIUC
Fixes: 290408d4a250 ("hugetlb: hugepage migration core")

should help. Maybe we should even tag that for stable?
-- 
Michal Hocko
SUSE Labs

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