linux-kernel - Re: [PATCH] writeback: Don't wait for completion in writeback_inodes_sb

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Date:	Wed, 13 Jul 2011 00:37:15 +0200
From:	Jan Kara <jack@...e.cz>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Jan Kara <jack@...e.cz>, Curt Wohlgemuth <curtw@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	fengguang.wu@...el.com
Subject: Re: [PATCH] writeback: Don't wait for completion in
 writeback_inodes_sb_nr

On Tue 12-07-11 06:41:32, Christoph Hellwig wrote:
> On Tue, Jul 12, 2011 at 12:34:53PM +0200, Jan Kara wrote:
> > > All block device inodes sit on blockdev_superblock, we got rid of inodes
> > > without a superblock long time ago.
> >   Sure, we can easily iterate also blockdev_superblock. What I meant is
> > that blockdev_superblock will need a special handling since we otherwise
> > ignore pseudo superblocks...
> 
> Pseudo superblocks aren't ignored.  They are added to super_blocks like
> all others, and iterate_supers doesn't skip over them.  The problem
> is that blockdev_superblock doesn't have a proper s_bdi set, and thus
> gets skipped over by __sync_filesystem. 
  Yes. But even if it was not skipped writeback_inodes_sb() doesn't have
one flusher thread to kick to actually do the writeout (since each inode on
blockdev_superblock belongs to a different bdi). So it's perfectly fine we
skip blockdev_superblock.

  If we want to fix the problem something like attached patch should do.
Comments?

								Honza

PS: While testing the patch, I've noticed that block device can have any
dirty data only if it is still open (__blkdev_put() writes all dirty pages)
so that somehow limits how much people can be burned by sync not writing
out block devices...
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR

View attachment "0001-vfs-Make-sync-1-writeout-also-block-device-inodes.patch" of type "text/x-patch" (4610 bytes)