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

[Roman Gushchin <guro@fb.com>]

On Mon, Mar 09, 2020 at 04:27:33PM -0700, Andrew Morton wrote:
> 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?

Good question. Let me double-check it.

Thanks!