Re: [PATCH v2] mm: hugetlb: optionally allocate gigantic hugepages using cma

[Rik van Riel]

On Tue, 2020-03-10 at 13:11 -0700, Mike Kravetz wrote:
> On 3/10/20 12:46 PM, Rik van Riel wrote:
> > 
> > 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?
> 
> For hugetlb page reservations at boot today, pairs specifying size
> and
> quantity are put on the command line.  For example,
> hugepagesz=2M hugepages=512 hugepagesz=1G hugepages=64
> 
> We could do something similiar for CMA.
> hugepagesz=512M hugepages_cma=256 hugepagesz=1G hugepages_cma=64
> 
> That would make things much more complicated (implies separate CMA
> reservations per size) and may be overkill for the first
> implementation.
> 
> 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=  analogous to today's default_hugepagesz=.  Then
> hugepages_cma= is only associated with that default gigantic huge
> page
> size.
> 
> 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.

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