linux-kernel - Re: [PATCH] ksm: use FAULT_FLAG_ALLOW

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Date:	Mon, 21 Nov 2011 15:10:39 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Hillf Danton <dhillf@...il.com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH] ksm: use FAULT_FLAG_ALLOW_RETRY in breaking COW

On Sat, Nov 19, 2011 at 3:50 AM, Hillf Danton <dhillf@...il.com> wrote:
> The flag, FAULT_FLAG_ALLOW_RETRY, was introduced by the patch,
>
>        mm: retry page fault when blocking on disk transfer
>        commit: d065bd810b6deb67d4897a14bfe21f8eb526ba99
>
> for reducing mmap_sem hold times that are caused by waiting for disk
> transfers when accessing file mapped VMAs.
>
> To break COW, handle_mm_fault() is repeated with mmap_sem held, where
> the introduced flag could be used again.
>
> The straight way is to add changes in break_ksm(), but the function could be
> under write-mode mmap_sem, so it has to be dupilcated.
>
> Signed-off-by: Hillf Danton <dhillf@...il.com>

I have to concur with Hugh here - FAULT_FLAG_ALLOW_RETRY was
introduced to avoid holding mmap_sem while we block on a disk read,
but you shouldn't hit this case in the break COW case, so there seems
to be little point in adding the flag there.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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