linux-ext4 - Re: [PATCH -V2] ext4: Drop mapped buffer_head check during page

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Message-ID: <20090829022656.GK16732@mit.edu>
Date:	Fri, 28 Aug 2009 22:26:56 -0400
From:	Theodore Tso <tytso@....edu>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	cmm@...ibm.com, sandeen@...hat.com, linux-ext4@...r.kernel.org
Subject: Re: [PATCH -V2] ext4: Drop mapped buffer_head check during
	page_mkwrite

On Wed, Aug 26, 2009 at 10:53:16AM +0530, Aneesh Kumar K.V wrote:
> Inorder to check whether the buffer_heads are mapped we need
> to hold page lock. Otherwise a reclaim can cleanup the attached
> buffer_heads. Instead of taking page lock and check whether
> buffer_heads are mapped we let the write_begin/write_end callback
> does the equivalent. It does have a performance impact in that we
> are doing more work if we the buffer_heads are already mapped.

So I started looking at all of the work that we need to do in
write_begin/write_end; did you check both write paths depending on
whether we are using delayed allocation or not?  It would seem to me
that it might be better to use lock_page() and unlock_page() around
the check, since in many work loads the buffer heads will already be
mapped often, and it appears to me that write_begin() will end up
locking the page anyway.

Am I missing something?

						- Ted
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