On Tue, Aug 14, 2012 at 05:57:06PM +0900, Minchan Kim wrote: > Now cma reclaims too many pages by __reclaim_pages which says > following as > > * Reclaim enough pages to make sure that contiguous allocation > * will not starve the system. > > Starve? What does it starve the system? The function which allocate > free page for migration target would wake up kswapd and do direct reclaim > if needed during migration so system doesn't starve. > I thought this patch was overkill at the time it was introduced but didn't have a concrete reason to reject it when I commented on it https://lkml.org/lkml/2012/1/30/136 . Marek did want this and followed up with "contiguous allocations should have higher priority than others" which I took to mean that he was also ok with excessive reclaim. > Let remove __reclaim_pages and related function and fields. > That should be one patch and I don't object to it being removed as such but it's Marek's call. > I modified split_free_page slightly because I removed __reclaim_pages, > isolate_freepages_range can fail by split_free_page's watermark check. > It's very critical in CMA because it ends up failing alloc_contig_range. > This is a big change and should have been in a patch on its own. split_free_page checks watermarks because if the watermarks are not obeyed a zone can become fully allocated. This can cause a system to livelock under certain circumstances if a page cannot be allocated and a free page is required before other pages can be freed. > I think we don't need the check in case of CMA because CMA allocates > free pages by alloc_pages, not isolate_freepages_block in migrate_pages > so watermark is already checked in alloc_pages. It uses alloc_pages when migrating pages out of the CMA area but note that it uses isolate_freepages_block when allocating the CMA buffer when alloc_contig_range calls isolate_freepages_range isolate_freepages_range(unsigned long start_pfn, unsigned long end_pfn) { for (pfn = start_pfn; pfn < end_pfn; pfn += isolated) { isolated = isolate_freepages_block(pfn, block_end_pfn, &freelist, true); } map_pages(&freelist); } so the actual CMA allocation itself is not using alloc_pages. By removing the watermark check you allow the CMA to breach watermarks and puts the system at risk of livelock. I'm not keen on the split_free_page() change at all. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>