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

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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





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