Re: [PATCH 2/4] mm/gup: restrict CMA region by using allocation scope API

[Joonsoo Kim <js1304@gmail.com>]

2020년 7월 17일 (금) 오후 5:26, Michal Hocko <mhocko@kernel.org>님이 작성:
>
> On Fri 17-07-20 16:46:38, Joonsoo Kim wrote:
> > 2020년 7월 15일 (수) 오후 5:24, Michal Hocko <mhocko@kernel.org>님이 작성:
> > >
> > > On Wed 15-07-20 14:05:27, Joonsoo Kim wrote:
> > > > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > > >
> > > > We have well defined scope API to exclude CMA region.
> > > > Use it rather than manipulating gfp_mask manually. With this change,
> > > > we can now use __GFP_MOVABLE for gfp_mask and the ZONE_MOVABLE is also
> > > > searched by page allocator. For hugetlb, gfp_mask is redefined since
> > > > it has a regular allocation mask filter for migration target.
> > > >
> > > > Note that this can be considered as a fix for the commit 9a4e9f3b2d73
> > > > ("mm: update get_user_pages_longterm to migrate pages allocated from
> > > > CMA region"). However, "Fixes" tag isn't added here since it is just
> > > > suboptimal but it doesn't cause any problem.
> > >
> > > But it is breaking the contract that the longterm pins never end up in a
> > > cma managed memory. So I think Fixes tag is really due. I am not sure
> > > about stable backport. If the patch was the trivial move of
> >
> > Previous implementation is correct since longterm pins never end up in a CMA
> > managed memory with that implementation. It's just a different and suboptimal
> > implementation to exclude the CMA area. This is why I don't add the "Fixes"A
> > tag on the patch.
>
> But the current implementation calls memalloc_nocma_restore too early so
> __gu_longterm_locked will migrate pages possibly to CMA ranges as there
> is no GFP_MOVABLE restriction in place. Or am I missing something?

IIUC, calling memalloc_nocma_restore() too early doesn't cause the
actual problem.

Final check is done by check_and_migrate_cma_pages() which is outside of
scope API. If we find a CMA page in between the gup range here, the page is
migrated to the migration target page and this target page is allocated by
new_non_cma_page().

new_non_cma_page() try to allocate the page without __GFP_MOVABLE so
returned page will not be CMA area memory. Therefore, even if
memalloc_nocma_restore() is called early, there is no actual problem.

Thanks.