Hello, Vladimir. On Fri, Aug 28, 2015 at 07:36:11PM +0300, Vladimir Davydov wrote: > > * try_charge() can be invoked from any in-kernel allocation site and > > reclaim path may use considerable amount of stack. This can lead to > > stack overflows which are extremely difficult to reproduce. > > IMO this paragraph does not justify this patch at all, because one will > still invoke direct reclaim from try_charge() on hitting the hard limit. Ah... right, and we can't defer direct reclaim for hard limit. > > * If the allocation doesn't have __GFP_WAIT, direct reclaim is > > skipped. If a process performs only speculative allocations, it can > > blow way past the high limit. This is actually easily reproducible > > by simply doing "find /". VFS tries speculative !__GFP_WAIT > > allocations first, so as long as there's memory which can be > > consumed without blocking, it can keep allocating memory regardless > > of the high limit. > > I think there shouldn't normally occur a lot of !__GFP_WAIT allocations > in a row - they should still alternate with normal __GFP_WAIT > allocations. Yes, that means we can breach memory.high threshold for a > short period of time, but it isn't a hard limit, so it looks perfectly > fine to me. > > I tried to run `find /` over ext4 in a cgroup with memory.high set to > 32M and kmem accounting enabled. With such a setup memory.current never > got higher than 33152K, which is only 384K greater than the memory.high. > Which FS did you use? ext4. Here, it goes onto happily consuming hundreds of megabytes with limit set at 32M. We have quite a few places where !__GFP_WAIT allocations are performed speculatively in hot paths with fallback slow paths, so this is bound to happen somewhere. Thanks. -- tejun -- 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>