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

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 9 Mar 2020 15:32:16 -0700 Roman Gushchin <guro@fb.com> wrote:

> Commit 944d9fec8d7a ("hugetlb: add support for gigantic page allocation
> at runtime") has added the run-time allocation of gigantic pages. However
> it actually works only at early stages of the system loading, when
> the majority of memory is free. After some time the memory gets
> fragmented by non-movable pages, so the chances to find a contiguous
> 1 GB block are getting close to zero. Even dropping caches manually
> doesn't help a lot.
> 
> At large scale rebooting servers in order to allocate gigantic hugepages
> is quite expensive and complex. At the same time keeping some constant
> percentage of memory in reserved hugepages even if the workload isn't
> using it is a big waste: not all workloads can benefit from using 1 GB
> pages.
> 
> The following solution can solve the problem:
> 1) On boot time a dedicated cma area* is reserved. The size is passed
>    as a kernel argument.
> 2) Run-time allocations of gigantic hugepages are performed using the
>    cma allocator and the dedicated cma area
> 
> In this case gigantic hugepages can be allocated successfully with a
> high probability, however the memory isn't completely wasted if nobody
> is using 1GB hugepages: it can be used for pagecache, anon memory,
> THPs, etc.
> 
> * On a multi-node machine a per-node cma area is allocated on each node.
>   Following gigantic hugetlb allocation are using the first available
>   numa node if the mask isn't specified by a user.
> 
> Usage:
> 1) configure the kernel to allocate a cma area for hugetlb allocations:
>    pass hugetlb_cma=10G as a kernel argument
> 
> 2) allocate hugetlb pages as usual, e.g.
>    echo 10 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
> 
> If the option isn't enabled or the allocation of the cma area failed,
> the current behavior of the system is preserved.
> 
> Only x86 is covered by this patch, but it's trivial to extend it to
> cover other architectures as well.
> 

Sounds promising.

I'm not seeing any dependencies on CONFIG_CMA in there.  Does the code
actually compile if CONFIG_CMA=n?  If yes, then does it add unneeded
bloat?