linux-kernel - Re: [PATCH] mm: shmem: unlock valid page

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Message-ID: <CAJd=RBCpXUfxp+9wo5fk32K=_ojwb5yGO=CejN_eK_Ud=P6rUA@mail.gmail.com>
Date:	Mon, 5 Mar 2012 20:15:38 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Linux-MM <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: shmem: unlock valid page

On Mon, Mar 5, 2012 at 1:51 AM, Hugh Dickins <hughd@...gle.com> wrote:
> On Sun, 4 Mar 2012, Hillf Danton wrote:
>> In shmem_read_mapping_page_gfp() page is unlocked if no error returned,
>> so the unlocked page has to valid.
>>
>> To guarantee that validity, when getting page, success result is feed
>> back to caller only when page is valid.
>>
>> Signed-off-by: Hillf Danton <dhillf@...il.com>
>
> I don't understand your description, nor its relation to the patch.
>
> NAK to the patch: when no page has previously been allocated, the
> SGP_READ case avoids allocation and returns NULL - do_shmem_file_read
> then copies the ZERO_PAGE instead, avoiding lots of unnecessary memory
> allocation when reading a large sparse file.
>
Hi Hugh

Thanks for your review.

It was not well prepared as I missed SGP_CACHE.

-hd
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