On Wed, Jul 20, 2011 at 04:17:41PM -0500, Christoph Lameter wrote: > Hmmm... Maybe we can bypass the checks? > Maybe we should not. Watermarks should not just be ignored. They prevent the system deadlocking due to an inability allocate a page needed to free more memory. This patch allows allocations that are not high priority or atomic to succeed when the buddy lists are at the min watermark and would normally be throttled. Minimally, this patch increasing the risk of the locking up due to memory expiration. For example, a GFP_ATOMIC allocation can refill the per-cpu list with the pages then consumed by GFP_KERNEL allocations, next GFP_ATOMIC allocation refills again, gets consumed etc. It's even worse if it's PF_MEMALLOC allocations that are refilling the lists as they ignore watermarks. If this is happening on enough CPUs, it will cause trouble. At the very least, the performance benefit of such a change should be illustrated. Even if it's faster (and I'd expect it to be, watermark checks particularly at low memory are expensive), it may just mean the system occasionally runs very fast into a wall. Hence, the patch should be accompanied with tests showing that even under very high stress for a long period of time that it does not lock up and the changelog should include a *very* convincing description on why PF_MEMALLOC refilling the per-cpu lists to be consumed by low-priority users is not a problem. -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>