linux-kernel - Re: [PATCH] mm: fix is_pinnable

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Date:   Sun, 8 May 2022 02:31:51 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        John Dias <joaodias@...gle.com>
Subject: Re: [PATCH] mm: fix is_pinnable_page against on cma page

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