Re: [PATCH -v2] mm, memcg: sync allocation and memcg charge gfp flags for THP

[David Rientjes <rientjes@google.com>]

On Wed, 18 Mar 2015, Michal Hocko wrote:

> memcg currently uses hardcoded GFP_TRANSHUGE gfp flags for all THP
> charges. THP allocations, however, might be using different flags
> depending on /sys/kernel/mm/transparent_hugepage/{,khugepaged/}defrag
> and the current allocation context.
> 
> The primary difference is that defrag configured to "madvise" value will
> clear __GFP_WAIT flag from the core gfp mask to make the allocation
> lighter for all mappings which are not backed by VM_HUGEPAGE vmas.
> If memcg charge path ignores this fact we will get light allocation but
> the a potential memcg reclaim would kill the whole point of the
> configuration.
> 
> Fix the mismatch by providing the same gfp mask used for the
> allocation to the charge functions. This is quite easy for all
> paths except for hugepaged kernel thread with !CONFIG_NUMA which is
> doing a pre-allocation long before the allocated page is used in
> collapse_huge_page via khugepaged_alloc_page. To prevent from cluttering
> the whole code path from khugepaged_do_scan we simply return the current
> flags as per khugepaged_defrag() value which might have changed since
> the preallocation. If somebody changed the value of the knob we would
> charge differently but this shouldn't happen often and it is definitely
> not critical because it would only lead to a reduced success rate of
> one-off THP promotion.
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Michal Hocko <mhocko@suse.cz>

Acked-by: David Rientjes <rientjes@google.com>

I'm slightly surprised that this issue never got reported before.