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

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Date:   Tue, 10 Mar 2020 10:30:56 -0700
From:   Roman Gushchin <guro@...com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>, <linux-mm@...ck.org>,
        <kernel-team@...com>, <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...riel.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH v2] mm: hugetlb: optionally allocate gigantic hugepages
 using cma

On Tue, Mar 10, 2020 at 10:01:21AM +0100, Michal Hocko wrote:
> On Mon 09-03-20 17:25:24, Roman Gushchin wrote:
> [...]
> > 2) Run-time allocations of gigantic hugepages are performed using the
> >    cma allocator and the dedicated cma area
> 
> [...]
> > @@ -1237,6 +1246,23 @@ static struct page *alloc_gigantic_page(struct hstate *h, gfp_t gfp_mask,
> >  {
> >  	unsigned long nr_pages = 1UL << huge_page_order(h);
> >  
> > +	if (IS_ENABLED(CONFIG_CMA) && hugetlb_cma[0]) {
> > +		struct page *page;
> > +		int nid;
> > +
> > +		for_each_node_mask(nid, *nodemask) {
> > +			if (!hugetlb_cma[nid])
> > +				break;
> > +
> > +			page = cma_alloc(hugetlb_cma[nid], nr_pages,
> > +					 huge_page_order(h), true);
> > +			if (page)
> > +				return page;
> > +		}
> > +
> > +		return NULL;
> 
> Is there any strong reason why the alloaction annot fallback to non-CMA
> allocator when the cma is depleted?

The reason is that that gigantic pages allocated using cma require
a special handling on releasing. It's solvable by using an additional
page flag, but because the current code is usually not working except
a short time just after the system start, I don't think it's worth it.

But I do not have a strong opinion here.

Thanks!