On Wed, Aug 07, 2013 at 06:14:37PM +0200, Andrea Arcangeli wrote: > Hi Mel, > > On Wed, Aug 07, 2013 at 04:42:01PM +0100, Mel Gorman wrote: > > On Fri, Aug 02, 2013 at 06:06:32PM +0200, Andrea Arcangeli wrote: > > > The min wmark should be satisfied with just 1 hugepage. > > > > This depends on the size of the machine and if THP is enabled or not > > (which adjusts min_free_kbytes). I expect that it is generally true but > > wonder how often it is true on something like ARM which does high-order > > allocators for stack. > > I exclude ARM is allocating stacks with GFP_ATOMIC, or how could it be > reliable? I assumed they were GFP_KERNEL allocations. High-order atomic allocations would be jumbo frame allocations. Anyway, my general concern is that min watermark can be smaller than 1 hugepage on some platforms and making assumptions about the watermarks and their relative size in comparison to THP seems dangerous. If it is possible that ((low - min) > requested_size) then a high-order allocation from the allocator slowpath will be allowed to go below the min watermark which is not expected. > If it's not an atomic allocation, it should make no > difference as it wouldn't be allowed to eat from the reservation below > MIN anyway, just the area between LOW and MIN matters, no? > > > It would be hard to hit but you may be able to trigger this warning if > > > > process a process b > > read min watermark > > increase min_free_kbytes > > __zone_watermark_ok > > > > > > > > > > if (min < 0) > > return false; > > > > ? > > Correct, this is why it's a WARN_ON and not a BUG_ON. if (WARN_ON(min < 0)) return false; ? Seems odd to just fall through and make decisions based on a negative watermark. It'll erronously return true when the revised watermark should have returned false. As it is likely due to a min_free_kbytes or hot-add event then it probably does not matter a great deal. I'm not massively pushed and the WARN_ON is fine if you like. It just struck me as being strange looking. -- 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>