linux-kernel - Re: unified page and buffer cache?

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Message-ID: <4BE4359E.9020400@cfl.rr.com>
Date:	Fri, 7 May 2010 11:45:34 -0400
From:	Phillip Susi <psusi@....rr.com>
To:	Matthew Wilcox <matthew@....cx>
CC:	linux-fsdevel@...r.kernel.org,
	Linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: unified page and buffer cache?

On 5/7/2010 9:53 AM, Matthew Wilcox wrote:
> The problem you're seeing is aliasing in the page cache, not a failed
> unification of the buffer and page caches.  Pages are addressed by
> (mapping, offset).  Each inode generally has its own mapping.  Depending
> on the file system, directories may be addressed by their own inode's
> mapping, or by the block device's mapping.

If there are two mappings that don't know about each other, then the
caches don't seem very unified to me.  If I write to the file and that
data sits in the mapping for the inode, then I read the corresponding
blocks though the block device, and it has a different mapping, then I
read the old data off the disk instead of the new data in the cache.  I
thought that this exact problem had been fixed long ago.

> Resolving aliasing would be horribly expensive, so it's unlikely to
> happen.

Back to the drawing board I guess.  Maybe ext could be fixed to use an
inode mapping for directories instead of relying on the block device
mapping, then I could readahead() the directory instead of having to go
to the block device at all.
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