Re: [PATCH 6/6] vmscan: Do not writeback pages in direct reclaim

[Mel Gorman <mel@csn.ul.ie>]

On Thu, Jun 10, 2010 at 11:17:06PM -0700, Andrew Morton wrote:
> On Tue,  8 Jun 2010 10:02:25 +0100 Mel Gorman <mel@csn.ul.ie> wrote:
> 
> > When memory is under enough pressure, a process may enter direct
> > reclaim to free pages in the same manner kswapd does. If a dirty page is
> > encountered during the scan, this page is written to backing storage using
> > mapping->writepage. This can result in very deep call stacks, particularly
> > if the target storage or filesystem are complex. It has already been observed
> > on XFS that the stack overflows but the problem is not XFS-specific.
> > 
> > This patch prevents direct reclaim writing back pages by not setting
> > may_writepage in scan_control. Instead, dirty pages are placed back on the
> > LRU lists for either background writing by the BDI threads or kswapd. If
> > in direct lumpy reclaim and dirty pages are encountered, the process will
> > kick the background flushter threads before trying again.
> > 
> 
> This wouldn't have worked at all well back in the days when you could
> dirty all memory with MAP_SHARED. 

Yes, it would have been a bucket of fail.

> The balance_dirty_pages() calls on
> the fault path will now save us but if for some reason we were ever to
> revert those, we'd need to revert this change too, I suspect.
> 

Quite likely.

> As it stands, it would be wildly incautious to make a change like
> this without first working out why we're pulling so many dirty pages
> off the LRU tail, and fixing that.
> 

Ok, I have a series prepared for testing that is in three parts.

Patches 1-4: tracepoints to gather how many dirty pages there really are
	being written out on the LRU
Patches 5-10: reduce the stack usage in page reclaim
Patches 9-10: Avoid writing out pages from direct reclaim and instead
	kicking background flushers to do the writing

Patches 1-4 on its own should an accurate view of how many dirty pages are
really being written back and if it's a real problem or not.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab