From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Subject: Re: Delayed allocation and page_lock vs transaction start ordering
Date: Tue, 27 May 2008 20:41:28 +0530
Message-ID: <20080527151128.GA13237@skywalker>
References: <20080415161430.GC28699@duck.suse.cz> <20080521082109.GA18746@skywalker> <20080526172124.GK32407@duck.suse.cz> <20080526180043.GB14718@skywalker> <20080527124312.GG5178@duck.suse.cz>
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Cc: linux-ext4@vger.kernel.org, sandeen@redhat.com
To: Jan Kara <jack@suse.cz>
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In-Reply-To: <20080527124312.GG5178@duck.suse.cz>
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On Tue, May 27, 2008 at 02:43:12PM +0200, Jan Kara wrote:
> On Mon 26-05-08 23:30:43, Aneesh Kumar K.V wrote:
> 
> > I have got another question now related to page_mkwrite. AFAIU writepage
> > writeout dirty buffer_heads. It also looks at whether the pages are
> > dirty or not. In the page_mkwrite callback both are not true. ie we call
> > set_page_dirty from do_wp_page after calling page_mkwrite. I haven't
> > verified whether the above is correct or not. Just thinking reading the
> > code.
>   Writepage call itself doesn't look at whether the page is dirty or not -
> that flag is already cleared when writepage is called. You are right that
> the page is marked dirty only after page_mkwrite is called - the meaning of
> page_mkwrite() call is roughly "someone wants to do the first write to this
> page via mmap, prepare filesystem for that". But we don't really care
> whether the page is dirty or not - we know it carries correct data (it is
> uptodate) and so we can write it if we want (and need).
> 


I am looking at  __block_write_full_page and we have

if (!buffer_mapped(bh) && buffer_dirty(bh)) {
	WARN_ON(bh->b_size != blocksize);
	err = get_block(inode, block, bh, 1);
	if (err)

ie, we do get_block only if the buffer_head is dirty. So I am bit
doubtful whether we are actually allocating blocks via page_mkwrite.

-aneesh