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@xxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
