linux-ext4 - Re: [PATCH] VFS: Pagecache usage optimization onpagesize!=blocksize environment

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Date:	Fri, 11 Jul 2008 16:39:01 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
Cc:	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] VFS: Pagecache usage optimization onpagesize!=blocksize
 environment

On Tue, 27 May 2008 18:34:02 +0900 Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp> wrote:

> When we read some part of a file through pagecache, if there is a pagecache 
> of corresponding index but this page is not uptodate, read IO is issued and
> this page will be uptodate.
> I think this is good for pagesize == blocksize environment but there is room
> for improvement on pagesize != blocksize environment. Because in this case
> a page can have multiple buffers and even if a page is not uptodate, some buffers 
> can be uptodate. So I suggest that when all buffers which correspond to a part
> of a file that we want to read are uptodate, use this pagecache and copy data
> from this pagecache to user buffer even if a page is not uptodate. This can
> reduce read IO and improve system throughput.
> 
> v2: add new address_space_operations member is_partially_uptodate, and 
>      block_is_partially_uptodate was registered to ext2/3/4's aops.
>      modify do_generic_file_read to use this aops callback.
> v3: use unsigned instead of unsigned long in block_is_partially_uptodate.
>      cleaned up and simplified page buffer iteration code in block_is_partially_uptodate.
> 
> Signed-off-by :Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>

ext4 has now gained a new set of address_space_operations:
ext4_da_aops.  I put a

        .is_partially_uptodate  = block_is_partially_uptodate,

into there as well, but I have no clue whether it will work OK.

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