From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Subject: Re: [PATCH 03/11] vfs: Add better VFS support for page_mkwrite
	when blocksize < pagesize
Date: Wed, 27 May 2009 22:15:04 +0530
Message-ID: <20090527164504.GC19989@skywalker>
References: <1243429268-3028-1-git-send-email-jack@suse.cz> <1243429268-3028-4-git-send-email-jack@suse.cz> <20090527160005.GA14035@dhcp231-156.rdu.redhat.com>
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Cc: Jan Kara <jack@suse.cz>, LKML <linux-kernel@vger.kernel.org>,
	npiggin@suse.de, linux-ext4@vger.kernel.org
To: Josef Bacik <josef@redhat.com>
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In-Reply-To: <20090527160005.GA14035@dhcp231-156.rdu.redhat.com>
Sender: linux-ext4-owner@vger.kernel.org

On Wed, May 27, 2009 at 12:00:06PM -0400, Josef Bacik wrote:
> On Wed, May 27, 2009 at 03:01:00PM +0200, Jan Kara wrote:
.....
....

> >  		if (offset > inode->i_sb->s_maxbytes)
> >  			goto out_big;
> > -		i_size_write(inode, offset);
> > +		do_extend_i_size(inode, offset, 0);
> >  	} else {
> >  		struct address_space *mapping = inode->i_mapping;
> >
> 
> Sorry if I'm being a bit dense, I'm just kind of confused.  In the case you
> outlined, we only allocate one block, as we should, but then the truncate
> extends i_size so that when writepage() comes along, its valid to write the
> whole page out, except we didn't allocate blocks for the whole page sine
> blocksize < pagesize.

We can't do block allocation in writepage because we can't handler
ENOSPC during writepage. So the patch attempt to make sure we do
all block allocation in the process context. For mmap we should
do it in page_mkwrite and for write in write_begin.


> 
> But if we didn't extend i_size, writepage would only have written the block's
> worth of data correct?  If thats the case, why don't we just make it so that
> happens in this case as well? 

That is what is done in the patch i posted
http://article.gmane.org/gmane.comp.file-systems.ext4/13468

But that is not just enough. We also need to make sure we get a
page_fault when we try to write at another offset via mmap address so
that we can do block allocation via page_mkwrite. To do that all code
path that extend i_size should mark the page write protect.


>Technically only one part of the page is dirty,
> so we just need to write that out, not the whole thing.  I assume there is a way
> to do this, since it presumably happens in the case where i_size < page size.
> If thats not possible then I guess this way is a good answer, it just seems
> overly complicated to me.


__block_write_full_page already does that. It looks at the last_block

> 
> Also, on the actualy patch, you only check the return value of
> block_lock_hole_extend in block_extend_i_size, but sinze
> block_unlock_hole_extend doesn't really do anything if we didn't do anything in
> block_lock_hole_extend, you might as well make it a void as well, since you
> unconditionally lock/unlock in every other instance.  Thanks,
> 
> Josef
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-aneesh