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

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Message-ID: <20230612164205.GA3704@monkey>
Date:   Mon, 12 Jun 2023 09:42:05 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     David Rientjes <rientjes@...gle.com>,
        Josh Triplett <josh@...htriplett.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 06/12/23 11:12, David Hildenbrand wrote:
> 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 ...
> 

Agree with David(s).  Such an option should really be decided by a sysadmin.

Any reason why liburing can not use THP?  Seems like that would provide the
desired functionality.
-- 
Mike Kravetz