linux-kernel - Re: [PATCH] writeback: Fix broken sync writeback

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Date:	Wed, 17 Feb 2010 02:33:37 +0100
From:	Jan Kara <jack@...e.cz>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jan Kara <jack@...e.cz>, Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	jengelh@...ozas.de, stable@...nel.org, gregkh@...e.de
Subject: Re: [PATCH] writeback: Fix broken sync writeback

On Tue 16-02-10 15:34:01, Linus Torvalds wrote:
> On Wed, 17 Feb 2010, Jan Kara wrote:
> >
> >   The IO size actually does matter for performance because if you switch
> > after 4 MB (current value of MAX_WRITEBACK_PAGES) to writing another inode,
> 
> No.
> 
> Dammit, read the code.
> 
> That's my whole _point_. Look at the for-loop.
> 
> We DO NOT SWITCH to another inode, because we just continue in the 
> for-loop.
> 
> This is why I think your patch is crap. You clearly haven't even read the 
> code, the patch makes no sense, and there must be something else going on 
> than what you _claim_ is going on.
  I've read the code. Maybe I'm missing something but look:
writeback_inodes_wb(nr_to_write = 1024)
  -> queue_io() - queues inodes from wb->b_dirty list to wb->b_io list
  ...
  writeback_single_inode()
    ...writes 1024 pages.
    if we haven't written everything in the inode (more than 1024 dirty
    pages) we end up doing either requeue_io() or redirty_tail(). In the
    first case the inode is put to b_more_io list, in the second case to
    the tail of b_dirty list. In either case it will not receive further
    writeout until we go through all other members of current b_io list.

  So I claim we currently *do* switch to another inode after 4 MB. That
is a fact.

  I *think* it is by design - mainly to avoid the situation where someone
continuously writes a huge file and kupdate or pdflush would never get to
writing other files with dirty data (at least that's impression I've built
over the years - heck, even 2.6.16 seems to have this redirty_tail logic
with a comment about the above livelock).

  I do find this design broken as well as you likely do and think that the
livelock issue described in the above paragraph should be solved differently
(e.g. by http://lkml.org/lkml/2010/2/11/321) but that's not a quick fix.

  The question is what to do now for 2.6.33 and 2.6.32-stable. Personally,
I think that changing the writeback logic so that it does not switch inodes
after 4 MB is too risky for these two kernels. So with the above
explanation would you accept some fix along the lines of original Jens'
fix?

								Honza

-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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