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

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Message-ID: <AANLkTin4-u8aA-KBA60xAmeBDg9BUvRp2sUieTkDcRhb@mail.gmail.com>
Date:	Mon, 13 Dec 2010 17:26:27 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...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 Mon, Dec 13, 2010 at 5:05 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Mon, 13 Dec 2010 16:51:40 -0800
> Michel Lespinasse <walken@...gle.com> wrote:
>> Andrew, should I amend my patches to remove the rwsem_is_contended() code ?
>> This would involve:
>> - remove rwsem-implement-rwsem_is_contended.patch and
>>   x86-rwsem-more-precise-rwsem_is_contended-implementation.patch
>> - in mlock-do-not-hold-mmap_sem-for-extended-periods-of-time.patch,
>>   drop the one hunk making use of rwsem_is_contended (rest of the patch
>>   would still work without it)
>
> I think I fixed all that up.

Thanks!

>> - optionally, follow up patch to limit batch size to a constant
>>   in do_mlock_pages():
[... diff snipped ...]
>> I don't really prefer using a constant, but I'm not sure how else to make
>> Linus happy :)
>
> rwsem_is_contended() didn't seem so bad to me.
>
> Reading 1024 pages can still take a long time.  I can't immediately
> think of a better approach though.

Note that we're only concerned page cache hist here, as
__get_user_pages with non-NULL nonblocking argument will release
mmap_sem when blocking on disk. So time per page is somewhat constant
- we don't need to worry about disk seeks at least.

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