On Wed 15-08-18 15:16:52, Andrew Morton wrote:
[...]
> From: Vlastimil Babka <vbabka@xxxxxxx>
> 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/20180612122624.8045-1-vbabka@xxxxxxx
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
The code is quite subtle and we have a bad history of copying stuff
without rethinking whether the code still is needed. Which is sad and a
clear sign that the code is too complex. I cannot say this change
doesn't have any subtle side effects but it makes the intention clear at
least so I _think_ it is good to go. If we find some unintended side
effects we should simply rethink the whole reset zonelist thing.
That being said
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>
> mm/page_alloc.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> --- a/mm/page_alloc.c~mm-page_alloc-actually-ignore-mempolicies-for-high-priority-allocations
> +++ a/mm/page_alloc.c
> @@ -4165,11 +4165,12 @@ retry:
> alloc_flags = reserve_flags;
>
> /*
> - * Reset the zonelist iterators if memory policies can be ignored.
> - * These allocations are high priority and system rather than user
> - * orientated.
> + * Reset the nodemask and zonelist iterators if memory policies can be
> + * ignored. These allocations are high priority and system rather than
> + * user oriented.
> */
> if (!(alloc_flags & ALLOC_CPUSET) || reserve_flags) {
> + ac->nodemask = NULL;
> ac->preferred_zoneref = first_zones_zonelist(ac->zonelist,
> ac->high_zoneidx, ac->nodemask);
> }
> _
>
--
Michal Hocko
SUSE Labs
