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

[Mike Kravetz]

On 3/10/20 1:15 PM, Rik van Riel wrote:
> 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.

True, there is no technical reason.

I was only trying to simplify the setup and answer the outstanding questions.
- What alignment to use for reservations?
- What is minimum size of reservations?

If only one gigantic page size is supported, the answer is simple.  In any
case, I think input from arch specific code will be needed.
-- 
Mike Kravetz

> 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.