linux-kernel - Re: [PATCH] hugetlb: detect race if fail to COW

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Message-ID: <CAJd=RBCOK9tis-bF87Csn70miRDqLtCUiZmDH2hnc8i_9+KtNw@mail.gmail.com>
Date:	Fri, 18 Nov 2011 23:23:12 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <jweiner@...hat.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hugetlb: detect race if fail to COW

On Fri, Nov 18, 2011 at 11:07 PM, Michal Hocko <mhocko@...e.cz> wrote:
> On Fri 18-11-11 22:04:37, Hillf Danton wrote:
>> In the error path that we fail to allocate new huge page, before try again, we
>> have to check race since page_table_lock is re-acquired.
>
> I do not think we can race here because we are serialized by
> hugetlb_instantiation_mutex AFAIU. Without this lock, however, we could
> fall into avoidcopy and shortcut despite the fact that other thread has
> already did the job.
>
> The mutex usage is not obvious in hugetlb_cow so maybe we want to be
> explicit about it (either a comment or do the recheck).
>

Then the following check is unnecessary, no?

Thanks,
Hillf

	/*
	 * Retake the page_table_lock to check for racing updates
	 * before the page tables are altered
	 */
	spin_lock(&mm->page_table_lock);
	ptep = huge_pte_offset(mm, address & huge_page_mask(h));
	if (likely(pte_same(huge_ptep_get(ptep), pte))) {
		/* Break COW */
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