Re: [PATCH v5 2/2] mm: hugetlb: optionally allocate gigantic hugepages using cma

[Roman Gushchin <guro@fb.com>]

On Tue, Apr 07, 2020 at 08:15:46PM +0200, Michal Hocko wrote:
> On Tue 07-04-20 09:38:40, Roman Gushchin 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.
> > 
> > x86 and arm-64 are covered by this patch, other architectures can be
> > trivially added later.
> > 
> > The patch contains clean-ups and fixes proposed and implemented by
> > Aslan Bakirov and Randy Dunlap. It also contains ideas and suggestions
> > proposed by Rik van Riel, Michal Hocko and Mike Kravetz. Thanks!
> > 
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> > Tested-by: Andreas Schaufler <andreas.schaufler@gmx.de>
> > Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
> > Acked-by: Michal Hocko <mhocko@kernel.org>
> > Cc: Aslan Bakirov <aslan@fb.com>
> > Cc: Randy Dunlap <rdunlap@infradead.org>
> > Cc: Rik van Riel <riel@surriel.com>
> > Cc: Joonsoo Kim <js1304@gmail.com>
> 
> Thanks a lot for addressing my review feedback!

Actually thank you for looking at it!

Andrew, can you, please, pick this version up instead of current patches and
fixes in the mm tree?
It will also fix the build on the next tree.

Thanks!