linux-kernel - Re: [PATCH v1] mm: hugetlb: call huge_pte

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Message-ID: <alpine.DEB.2.10.1511301409380.10460@chino.kir.corp.google.com>
Date:	Mon, 30 Nov 2015 14:10:16 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Mel Gorman <mgorman@...e.de>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Hillf Danton <hillf.zj@...baba-inc.com>,
	Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH v1] mm: hugetlb: call huge_pte_alloc() only if ptep is
 null

On Thu, 26 Nov 2015, Naoya Horiguchi wrote:

> Currently at the beginning of hugetlb_fault(), we call huge_pte_offset()
> and check whether the obtained *ptep is a migration/hwpoison entry or not.
> And if not, then we get to call huge_pte_alloc(). This is racy because the
> *ptep could turn into migration/hwpoison entry after the huge_pte_offset()
> check. This race results in BUG_ON in huge_pte_alloc().
> 
> We don't have to call huge_pte_alloc() when the huge_pte_offset() returns
> non-NULL, so let's fix this bug with moving the code into else block.
> 
> Note that the *ptep could turn into a migration/hwpoison entry after
> this block, but that's not a problem because we have another !pte_present
> check later (we never go into hugetlb_no_page() in that case.)
> 
> Fixes: 290408d4a250 ("hugetlb: hugepage migration core")
> Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Cc: <stable@...r.kernel.org> [2.6.36+]

Acked-by: David Rientjes <rientjes@...gle.com>

It would be nice to provide a sample of the BUG_ON() output in the commit 
message, however, so people can quickly find this if greping for what they 
just saw kill their machine.
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