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Subject: Re: [PATCH v2] mm: hugetlb: optionally allocate gigantic hugepages
 using cma
From:   Rik van Riel <riel@surriel.com>
To:     Mike Kravetz <mike.kravetz@oracle.com>,
        Michal Hocko <mhocko@kernel.org>, Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>, linux-mm@kvack.org,
        kernel-team@fb.com, linux-kernel@vger.kernel.org
Date:   Tue, 10 Mar 2020 16:15:18 -0400
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On Tue, 2020-03-10 at 13:11 -0700, Mike Kravetz wrote:
> On 3/10/20 12:46 PM, Rik van Riel wrote:
> >=20
> > How would that work for architectures that have multiple
> > possible hugetlbfs gigantic page sizes, where the admin
> > can allocate different numbers of differently sized pages
> > after bootup?
>=20
> For hugetlb page reservations at boot today, pairs specifying size
> and
> quantity are put on the command line.  For example,
> hugepagesz=3D2M hugepages=3D512 hugepagesz=3D1G hugepages=3D64
>=20
> We could do something similiar for CMA.
> hugepagesz=3D512M hugepages_cma=3D256 hugepagesz=3D1G hugepages_cma=3D64
>=20
> That would make things much more complicated (implies separate CMA
> reservations per size) and may be overkill for the first
> implementation.
>=20
> Perhaps we limit CMA reservations to one gigantic huge page
> size.  The
> architectures would need to define the default and there could be a
> command line option to override.  Something like,
> default_cmapagesz=3D  analogous to today's default_hugepagesz=3D.  Then
> hugepages_cma=3D is only associated with that default gigantic huge
> page
> size.
>=20
> The more I think about it, the more I like limiting CMA reservations
> to
> only one gigantic huge page size (per arch).

Why, though?

The cma_alloc function can return allocations of different
sizes at the same time.

There is no limitation in the underlying code that would stop
a user from allocating hugepages of different sizes through
sysfs.

Allowing the system administrator to allocate a little extra
memory for the CMA pool could also allow us to work around
initial issues of compaction/migration failing to move some
of the pages, while we play whack-a-mole with the last corner
cases.

--=20
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