linux-kernel - Re: [PATCH v2] mm: hugetlb: optionally allocate gigantic hugepages using cma

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Date:   Sun, 15 Mar 2020 21:08:33 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Michal Hocko <mhocko@...nel.org>, Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
        kernel-team@...com, linux-kernel@...r.kernel.org,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH v2] mm: hugetlb: optionally allocate gigantic hugepages
 using cma

On Tue, 2020-03-10 at 18:37 +0100, Michal Hocko wrote:
> On Tue 10-03-20 10:25:59, Roman Gushchin wrote:
> > Well, so far I was focused on a particular case when the target cma
> > size
> > is significantly smaller than the total RAM size (~5-10%). What is
> > the right
> > thing to do here? Fallback to the current behavior if the requested
> > size is
> > more than x% of total memory? 1/2? How do you think?
> 
> I would start by excluding restricted kernel zones (<ZONE_NORMAL).
> Cutting off 1G of ZONE_DMA32 might be a real problem.

It looks like memblock_find_in_range_node(), which
is called from memblock_alloc_range_nid(), will already
do top-down allocation inside each node.

However, looking at that code some more, it has some
limitations that we might not want. Specifically, if
we want to allocate for example a 16GB CMA area, but
the node in question only has a 15GB available area
in one spot and a 1GB available area in another spot,
for example due to memory holes, the allocation will fail.

I wonder if it makes sense to have separate cma_declare_contiguous
calls for each 1GB page we set up. That way it will be easier
to round-robin between the ZONE_NORMAL zones in each node, and
also to avoid the ZONE_DMA32 and other special nodes on systems
where those are a relatively small part of memory.

I'll whip up a patch to do that.

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