linux-kernel - Re: [PATCH] mm: hugetlb: Add Kconfig option to set default nr_overcommit

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Message-ID: <fd1c3a84-d3e6-906f-57b0-25eae0933b68@redhat.com>
Date:   Mon, 12 Jun 2023 11:12:37 +0200
From:   David Hildenbrand <david@...hat.com>
To:     David Rientjes <rientjes@...gle.com>,
        Josh Triplett <josh@...htriplett.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] mm: hugetlb: Add Kconfig option to set default
 nr_overcommit_hugepages

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@...htriplett.org>
> 
> 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