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

[Andrew Morton <akpm@linux-foundation.org>]

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.  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.


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.