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

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Date:	Mon, 21 Nov 2011 14:23:32 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Hillf Danton <dhillf@...il.com>
cc:	Michel Lespinasse <walken@...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 Mon, 21 Nov 2011, Hillf Danton wrote:
> On Mon, Nov 21, 2011 at 12:16 PM, Hugh Dickins <hughd@...gle.com> wrote:
> 
> After reading your reply and the comments in break_ksm(), if the patch does
> not mess up
> 	"The important thing is to not let VM_MERGEABLE be cleared while any
> 	 such pages might remain in the area",
> and
> 	"because handle_mm_fault() may back out if there's
> 	 any difficulty e.g. if pte accessed bit gets updated concurrently",
> 
> then if the path in which lock_page_or_retry() is called is not involved,
> mmap_sem is not upped, so the patch has nearly same behavior with break_ksm.
> 
> And the overhead of the patch, I think, could match break_ksm.
> 
> With dozen cases of writers of mmap_sem in the mm directory, the patch looks
> more flexible in rare and rare corners.

But what's the point in enlarging the kernel, adding code to make
break_cow() look more complicated, when there's no way in which the
addition can make an improvement?

Adding in a FAULT_FLAG_ALLOW_RETRY flag is not enough for mmap_sem
to be dropped for retry: you'd need a lock_page_or_retry() on the
faulting path and I do not see that here - please point it out to
me if you can see it.

(And I'll be somewhat sceptical if you respond with patches adding
lock_page_or_retry() all over, in order to meet this objection!)

Hugh
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