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

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Message-ID: <CAO81RMYJ76kdQ0qVuSSrfxugrTKAveSP98XQ2QF+VdbtjzJ--w@mail.gmail.com>
Date:	Thu, 14 Jul 2011 09:29:34 -0700
From:	Curt Wohlgemuth <curtw@...gle.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Christoph Hellwig <hch@...radead.org>,
	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

Hi Jan:

On Tue, Jul 12, 2011 at 3:37 PM, Jan Kara <jack@...e.cz> wrote:
> 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?

Your patch looks good to me, in that it does hit all the bdevs with
both WB_SYNC_NONE and SYNC_ALL.  However, I still say that the call to
wakeup_flusher_threads() in sys_sync() is superfluous, at least as
long as writeback_inodes_sb() waits for completion of the work item
that it enqueues.

Thanks,
Curt

>
>                                                                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
>
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