linux-kernel - Re: [PATCH 1/6] mlock: only hold mmap_sem in shared mode when faulting in pages

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Message-ID: <AANLkTikXx4MgdPYWYNVj8cMOSHTHJEUHqKZ_q-P4jFYp@mail.gmail.com>
Date:	Thu, 9 Dec 2010 22:39:29 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Piggin <npiggin@...nel.dk>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] mlock: only hold mmap_sem in shared mode when
 faulting in pages

On Thu, Dec 9, 2010 at 10:11 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Wednesday, December 8, 2010, Michel Lespinasse <walken@...gle.com> wrote:
>> Yes, patch 1/6 changes the long hold time to be in read mode instead
>> of write mode, which is only a band-aid. But, this prepares for patch
>> 5/6, which releases mmap_sem whenever there is contention on it or
>> when blocking on disk reads.
>
> I have to say that I'm not a huge fan of that horribly kludgy
> contention check case.
>
> The "move page-in to read-locked sequence" and the changes to
> get_user_pages look fine, but the contention thing is just disgusting.
> I'd really like to see some other approach if at all possible.

Are you OK with the part of patch 5/6 that drops mmap_sem when
blocking on disk ?

This by itself brings mmap_sem hold time down to a few seconds. Plus,
I could add something to limit the interval passed to
__mlock_vma_pages_range to a thousand pages or so, so that the hold
time would then be bounded by that constant.

I think rwsem_is_contended() actually sounds better than fiddling with
constants, but OTOH maybe the mlock use case is not significant enough
to justify introducing that new API.

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