Re: [PATCH] mm: hugetlb: Add Kconfig option to set default nr_overcommit_hugepages

[David Hildenbrand]

On 11.06.23 07:20, David Rientjes wrote:
> On Fri, 9 Jun 2023, Josh Triplett wrote:
>
> > The default kernel configuration does not allow any huge page allocation
> > until after setting nr_hugepages or nr_overcommit_hugepages to a
> > non-zero value; without setting those, mmap attempts with MAP_HUGETLB
> > will always fail with -ENOMEM. nr_overcommit_hugepages allows userspace
> > to attempt to allocate huge pages at runtime, succeeding if the kernel
> > can find or assemble a free huge page.
> >
> > Provide a Kconfig option to make nr_overcommit_hugepages default to
> > unlimited, which permits userspace to always attempt huge page
> > allocation on a best-effort basis. This makes it easier and more
> > worthwhile for random applications and libraries to opportunistically
> > attempt MAP_HUGETLB allocations without special configuration.
> >
> > In particular, current versions of liburing with IORING_SETUP_NO_MMAP
> > attempt to allocate the rings in a huge page. This seems likely to lead
> > to more applications and libraries attempting to use huge pages.
> >
> > Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
>
> Why not do this in an initscript?
>
> Or, if absolutely necessary, a kernel command line parameter?
>
> A Kconfig option to set a default value to be ULONG_MAX seems strange if
> you can just write the value to procfs.

Agreed, not to mention that huge pages in some environment can cause 
trouble (some architectures -- or with gigantic huge pages --  don't 
support huge page migration and you can run into trouble with 
ZONE_MOVABLE or MIGRATE_CMA, because you'll end up "consuming" all 
memory for unmovable allocations in the system), and we shouldn't 
advocate the use of unlimited overcommit for huge pages ...

--
Cheers,

David / dhildenb