Re: [PATCH] mm: fix is_pinnable_page against on cma page

[David Hildenbrand <david@redhat.com>]

On 05.05.22 19:25, Peter Xu wrote:
> On Thu, May 05, 2022 at 10:00:07AM -0700, Mike Kravetz wrote:
>> Gigantic pages can only be migrated IF there is another (already allocated)
>> gigantic page available.  The routine to try and allocate a page 'on the fly'
>> for migration will fail if passed a gigantic size.  There 'might' be a free
>> pre-allocated gigantic page.  However, if the user set up CMA reserves for
>> gigantic page allocations it is likely the free gigantic page is also in CMA.
>> Therefore, it can not be used for this migration.  So, unless my reasoning
>> is wrong, FOLL_LONGTERM would almost always fail for gigantic pages in CMA.
> 
> I'm probably not familiar enough with CMA, but.. I just noticed that if CMA
> is destined to not be able to be pinned then maybe it'll lose quite a few
> scenarios where pinning is a possible use case.  It doesn't even need to be
> the major use case, but as long as it's possible (e.g. hypervisors hosting
> virtual machines with device assignment) it'll be a hard no to CMA, which
> seems to be a pity.
> 

Well, the same applies to ZONE_MOVABLE as well, unfortunately.
Eventually, we might want to disable placing hugetlb pages on CMA areas
if it turns out to be a problem. In case of ZONE_MOVABLE we can already
fail "gracefully" when trying offlining (although that's really far from
beautiful).

-- 
Thanks,

David / dhildenb