linux-kernel - Re: per-bdi-throttling: synchronous writepage doesn't work correctly

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Message-Id: <1193937408.27652.326.camel@twins>
Date:	Thu, 01 Nov 2007 18:16:48 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	jdike@...toit.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: per-bdi-throttling: synchronous writepage doesn't work
	correctly

On Thu, 2007-11-01 at 18:09 +0100, Peter Zijlstra wrote:
> On Thu, 2007-11-01 at 18:00 +0100, Miklos Szeredi wrote:
> > > > Hi,
> > > > 
> > > > It looks like bdi_thresh will always be zero if filesystem does
> > > > synchronous writepage, resulting in very poor write performance.
> > > > 
> > > > Hostfs (UML) is one such example, but there might be others.
> > > > 
> > > > The only solution I can think of is to add a set_page_writeback();
> > > > end_page_writeback() pair (or some reduced variant, that only does
> > > > the proportions magic).  But that means auditing quite a few
> > > > filesystems...
> > > 
> > > Ouch...
> > > 
> > > I take it there is no other function that is shared between all these
> > > writeout paths which we could stick a bdi_writeout_inc(bdi) in?
> > 
> > No, and you can't detect it from the callers either I think.
> 
> The page not having PG_writeback set on return is a hint, but not fool
> proof, it could be the device is just blazing fast.
> 
> I guess there is nothing to it but for me to grep writepage and manually
> look at all hits...

  writepage: called by the VM to write a dirty page to backing store.
      This may happen for data integrity reasons (i.e. 'sync'), or
      to free up memory (flush).  The difference can be seen in
      wbc->sync_mode.
      The PG_Dirty flag has been cleared and PageLocked is true.
      writepage should start writeout, should set PG_Writeback,
      and should make sure the page is unlocked, either synchronously
      or asynchronously when the write operation completes.

      If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
      try too hard if there are problems, and may choose to write out
      other pages from the mapping if that is easier (e.g. due to
      internal dependencies).  If it chooses not to start writeout, it
      should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
      calling ->writepage on that page.

      See the file "Locking" for more details.


The "should set PG_Writeback" bit threw me off I guess.

Anyway, do we want me to just stick in
bdi_writeout_inc(page->mapping->backing_dev_info) everywhere, or do we
want to dress this up in a nice API?

If so, any suggestions?

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