On Wed, Oct 12, 2016 at 03:16:27PM +0200, Michal Hocko wrote: > On Wed 12-10-16 11:43:37, Michal Hocko wrote: > > On Wed 12-10-16 14:55:24, Anshuman Khandual wrote: > [...] > > > Why we insist on __GFP_THISNODE ? > > > > AFAIU __GFP_THISNODE just overrides the given node to the policy > > nodemask in case the current node is not part of that node mask. In > > other words we are ignoring the given node and use what the policy says. > > I can see how this can be confusing especially when confronting the > > documentation: > > > > * __GFP_THISNODE forces the allocation to be satisified from the requested > > * node with no fallbacks or placement policy enforcements. > > You made me think and look into this deeper. I came to the conclusion > that this is actually a relict from the past. policy_zonelist is called > only from 3 places: > - huge_zonelist - never should do __GFP_THISNODE when going this path > - alloc_pages_vma - which shouldn't depend on __GFP_THISNODE either > - alloc_pages_current - which uses default_policy id __GFP_THISNODE is > used > > So AFAICS this is essentially a dead code or I am missing something. Mel > do you remember why we needed it in the past?
I don't recall a specific reason. It was likely due to confusion on my part at the time on the exact use of __GFP_THISNODE. The expectation is that flag is not used in fault paths or with policies. It's meant to enforce node-locality for kernel internal decisions such as the locality of slab pages and ensuring that a THP collapse from khugepaged is on the same node. -- Mel Gorman SUSE Labs