linux-kernel - Re: Is nobh code still useful?

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Date:	Thu, 17 Sep 2009 21:21:37 -0700
From:	Badari Pulavarty <pbadari@...ibm.com>
To:	Jan Kara <jack@...e.cz>
CC:	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Is nobh code still useful?

Jan Kara wrote:
>   Hi,
>
>   during my page_mkwrite() work, I've looked at who uses nobh_ versions of
> various functions in fs/buffer.c. It seems only ext2 and jfs use them. ext3
> uses them only from writepage() (which means we needn't attach buffers to a
> page when it was written via mmap in writeback mode) and ext4 tries to use
> them but in fact it's nop because it always attaches buffers to the page
> earlier. So it's not really widely used, there's quite some code to support
> it (including one page flag), and it also slightly complicates my
> page_mkwrite() fixes.
>   So I wanted to ask does somebody actually remember what it is good for?
> Buffer heads obviously consume some memory so was that the reason? OTOH we
> have to map the page whenever we write to it or send it to disk via
> writepage().
>
>   
Originally it was supported on ext2. I added support nobh support for 
ext3. At that time, the main
issue/complaint was that, these bufferheads consume memory from 
ZONE_NORMAL causing
memory pressure on 32-bit (i386) configurations.

Thanks,
Badari


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