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

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Message-ID: <20090826022445.GA32712@mit.edu>
Date:	Tue, 25 Aug 2009 22:24:45 -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] ext4: Drop mapped buffer_head check during page_mkwrite

On Tue, Aug 25, 2009 at 07:52:59PM +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.

I'm not sure I understand the commit description.  From the patch you
are removing the check to see if all of the buffers are mapped.  But
the patch isn't moving the check to ext4_write_begin() or
ext4_write_end().  Are you saying the check is already in
ext4_write_begin()?  It doesn't seem to be in ext4_write_end().  

I see that we do call write_page_buffers() in ext4_write_begin(), and
in do_journal_get_write_access() we do check to see if the buffers are
present.  But if they aren't, we don't return an error; we just fail
request journal write access for the buffer head.

Am I missing something?  This patch doesn't feel complete, or the
commit description is very confusing....

						- Ted
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