linux-kernel - Re: [PATCH -alternative] mm: hugetlbfs: Close race during teardown of hugetlbfs shared page tables V2 (resend)

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Date:	Thu, 26 Jul 2012 13:42:26 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Hugh Dickins <hughd@...gle.com>
CC:	Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.cz>,
	Linux-MM <linux-mm@...ck.org>,
	David Gibson <david@...son.dropbear.id.au>,
	Ken Chen <kenchen@...gle.com>,
	Cong Wang <xiyou.wangcong@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Larry Woodman <lwoodman@...hat.com>
Subject: Re: [PATCH -alternative] mm: hugetlbfs: Close race during teardown
 of hugetlbfs shared page tables V2 (resend)

On 07/23/2012 12:04 AM, Hugh Dickins wrote:

> Please don't be upset if I say that I don't like either of your patches.
> Mainly for obvious reasons - I don't like Mel's because anything with
> trylock retries and nested spinlocks worries me before I can even start
> to think about it; and I don't like Michal's for the same reason as Mel,
> that it spreads more change around in common paths than we would like.

I have a naive question.

In huge_pmd_share, we protect ourselves by taking
the mapping->i_mmap_mutex.

Is there any reason we could not take the i_mmap_mutex
in the huge_pmd_unshare path?

I see that hugetlb_change_protection already takes that
lock. Is there something preventing __unmap_hugepage_range
from also taking mapping->i_mmap_mutex?

That way the sharing and the unsharing code are
protected by the same, per shm segment, lock.
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