From: Chris Mason <mason@suse.com>
To: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Subject: Re: o_sync in vfat driver
Date: Thu, 2 Mar 2006 12:01:31 -0500
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Cc: Andrew Morton <akpm@osdl.org>, col-pepper@piments.com,
       linux-kernel@vger.kernel.org
References: <op.s5lrw0hrj68xd1@mail.piments.com> <200603020845.10083.mason@suse.com> <87u0ahszxa.fsf@duaron.myhome.or.jp>
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On Thursday 02 March 2006 09:07, OGAWA Hirofumi wrote:
> Chris Mason <mason@suse.com> writes:
> > filemap_fdatawrite() won't redirty the page.  It will wait on the pending
> > writeback.
>
> Umm... I'm looking the following code.
>
> +void
> +writeback_bdev(struct super_block *sb)
> +{
> +	struct address_space *mapping = sb->s_bdev->bd_inode->i_mapping;
> +	filemap_flush(mapping);
> +	blk_run_address_space(mapping);
> +}
> +EXPORT_SYMBOL_GPL(writeback_bdev);
>
> filemap_flush() is using WB_SYNC_NONE.
>
Ok, I thought you were asking about the code that called filemap_fdatawrite, 
which does wait.  filemap_flush is used on the underlying block device.  In 
the case of a page that is already under IO, the io is not cancelled but 
allowed to continue.

This is the desired result.  When you're doing a number of operations in 
sequence, each operation will start io on the block device.  If they used 
filemap_fdatawrite instead of filemap_flush, they would end up being 
synchronous.

-chris

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