Re: [PATCH v4 04/11] mm/hugetlb: make hugetlb migration callback CMA aware

[Joonsoo Kim]

On Tue, Jul 07, 2020 at 01:22:31PM +0200, Vlastimil Babka wrote:
> On 7/7/20 9:44 AM, js1304@xxxxxxxxx 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.
> > 
> > hugetlb page allocation process consists of two steps.  First is dequeing
> > from the pool.  Second is, if there is no available page on the queue,
> > allocating from the page allocator.
> > 
> > new_non_cma_page() can control allocation from the page allocator by
> > specifying correct gfp flag.  However, dequeing cannot be controlled until
> > now, so, new_non_cma_page() skips dequeing completely.  It is a suboptimal
> > since new_non_cma_page() cannot utilize hugetlb pages on the queue so this
> > patch tries to fix this situation.
> > 
> > This patch makes the deque function on hugetlb CMA aware and skip CMA
> > pages if newly added skip_cma argument is passed as true.
> 
> Hmm, can't you instead change dequeue_huge_page_node_exact() to test the PF_
> flag and avoid adding bool skip_cma everywhere?

Okay! Please check following patch.
> 
> I think that's what Michal suggested [1] except he said "the code already does
> by memalloc_nocma_{save,restore} API". It needs extending a bit though, AFAICS.
> __gup_longterm_locked() indeed does the save/restore, but restore comes before
> check_and_migrate_cma_pages() and thus new_non_cma_page() is called, so an
> adjustment is needed there, but that's all?
> 
> Hm the adjustment should be also done because save/restore is done around
> __get_user_pages_locked(), but check_and_migrate_cma_pages() also calls
> __get_user_pages_locked(), and that call not being between nocma save and
> restore is thus also a correctness issue?

Simply, I call memalloc_nocma_{save,restore} in new_non_cma_page(). It
would not cause any problem.

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