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Date:   Thu, 5 May 2022 13:25:53 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Mike Kravetz <mike.kravetz@oracle.com>
Cc:     Minchan Kim <minchan@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        John Hubbard <jhubbard@nvidia.com>,
        John Dias <joaodias@google.com>
Subject: Re: [PATCH] mm: fix is_pinnable_page against on cma page
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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.

-- 
Peter Xu