On Wed, Aug 15, 2018 at 03:16:52PM -0700, Andrew Morton wrote: > From: Vlastimil Babka <[email protected]> > Subject: mm, page_alloc: actually ignore mempolicies for high priority > allocations > > The __alloc_pages_slowpath() function has for a long time contained code > to ignore node restrictions from memory policies for high priority > allocations. The current code that resets the zonelist iterator however > does effectively nothing after commit 7810e6781e0f ("mm, page_alloc: do > not break __GFP_THISNODE by zonelist reset") removed a buggy zonelist > reset. Even before that commit, mempolicy restrictions were still not > ignored, as they are passed in ac->nodemask which is untouched by the > code. > > We can either remove the code, or make it work as intended. Since > ac->nodemask can be set from task's mempolicy via alloc_pages_current() > and thus also alloc_pages(), it may indeed affect kernel allocations, and > it makes sense to ignore it to allow progress for high priority > allocations. > > Thus, this patch resets ac->nodemask to NULL in such cases. This assumes > all callers can handle it (i.e. there are no guarantees as in the case of > __GFP_THISNODE) which seems to be the case. The same assumption is > already present in check_retry_cpuset() for some time. > > The expected effect is that high priority kernel allocations in the > context of userspace tasks (e.g. OOM victims) restricted by mempolicies > will have higher chance to succeed if they are restricted to nodes with > depleted memory, while there are other nodes with free memory left. > > > Ot's not a new intention, but for the first time the code will match the > intention, AFAICS. It was intended by commit 183f6371aac2 ("mm: ignore > mempolicies when using ALLOC_NO_WATERMARK") in v3.6 but I think it never > really worked, as mempolicy restriction was already encoded in nodemask, > not zonelist, at that time. > > So originally that was for ALLOC_NO_WATERMARK only. Then it was adjusted > by e46e7b77c909 ("mm, page_alloc: recalculate the preferred zoneref if the > context can ignore memory policies") and cd04ae1e2dc8 ("mm, oom: do not > rely on TIF_MEMDIE for memory reserves access") to the current state. So > even GFP_ATOMIC would now ignore mempolicies after the initial attempts > fail - if the code worked as people thought it does. > > Link: http://lkml.kernel.org/r/[email protected] > Signed-off-by: Vlastimil Babka <[email protected]> > Cc: Mel Gorman <[email protected]> > Cc: Michal Hocko <[email protected]> > Cc: David Rientjes <[email protected]> > Cc: Joonsoo Kim <[email protected]> > Signed-off-by: Andrew Morton <[email protected]>
FWIW, I thought I acked this already. Acked-by: Mel Gorman <[email protected]> -- Mel Gorman SUSE Labs
