Re: [PATCH v3 4/8] mm/hugetlb: make hugetlb migration callback CMA aware

[Michal Hocko]

On Fri 26-06-20 13:49:15, Joonsoo Kim wrote:
> 2020ë 6ì 25ì (ë) ìí 8:54, Michal Hocko <mhocko@xxxxxxxxxx>ëì ìì:
> >
> > On Tue 23-06-20 15:13:44, Joonsoo Kim wrote:
> > > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > >
> > > new_non_cma_page() in gup.c which try to allocate migration target page
> > > requires to allocate the new page that is not on the CMA area.
> > > new_non_cma_page() implements it by removing __GFP_MOVABLE flag. This way
> > > works well for THP page or normal page but not for hugetlb page.
> >
> > Could you explain why? I mean why cannot you simply remove __GFP_MOVABLE
> > flag when calling alloc_huge_page_nodemask and check for it in dequeue
> > path?
> 
> If we remove __GFP_MOVABLE when calling alloc_huge_page_nodemask, we cannot
> use the page in ZONE_MOVABLE on dequeing.
> 
> __GFP_MOVABLE is not only used for CMA selector but also used for zone
> selector.  If we clear it, we cannot use the page in the ZONE_MOVABLE
> even if it's not CMA pages.  For THP page or normal page allocation,
> there is no way to avoid this weakness without introducing another
> flag or argument. For me, introducing another flag or argument for
> these functions looks over-engineering so I don't change them and
> leave them as they are (removing __GFP_MOVABLE).
> 
> But, for alloc_huge_page_nodemask(), introducing a new argument
> doesn't seem to be a problem since it is not a general function but
> just a migration target allocation function.

I really do not see why hugetlb and only the dequeing part should be
special. This just leads to a confusion. From the code point of view it
makes perfect sense to opt out CMA regions for !__GFP_MOVABLE when
dequeing. So I would rather see a consistent behavior than a special
case deep in the hugetlb allocator layer.
-- 
Michal Hocko
SUSE Labs