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Date:   Tue, 10 Mar 2020 18:42:19 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Mike Kravetz <mike.kravetz@oracle.com>
Cc:     Roman Gushchin <guro@fb.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>, linux-mm@kvack.org,
        kernel-team@fb.com, linux-kernel@vger.kernel.org,
        Rik van Riel <riel@surriel.com>
Subject: Re: [PATCH v2] mm: hugetlb: optionally allocate gigantic hugepages
 using cma
Message-ID: <20200310174219.GY8447@dhcp22.suse.cz>
References: <20200310002524.2291595-1-guro@fb.com>
 <20200310084544.GY8447@dhcp22.suse.cz>
 <ce96c9e9-1082-df68-010e-b759d2ede69a@oracle.com>
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On Tue 10-03-20 10:38:24, Mike Kravetz wrote:
> On 3/10/20 1:45 AM, Michal Hocko wrote:
> > On Mon 09-03-20 17:25:24, Roman Gushchin wrote:
> <snip>
> >> +early_param("hugetlb_cma", cmdline_parse_hugetlb_cma);
> >> +
> >> +void __init hugetlb_cma_reserve(void)
> >> +{
> >> +	unsigned long totalpages = 0;
> >> +	unsigned long start_pfn, end_pfn;
> >> +	phys_addr_t size;
> >> +	int nid, i, res;
> >> +
> >> +	if (!hugetlb_cma_size && !hugetlb_cma_percent)
> >> +		return;
> >> +
> >> +	if (hugetlb_cma_percent) {
> >> +		for_each_mem_pfn_range(i, MAX_NUMNODES, &start_pfn, &end_pfn,
> >> +				       NULL)
> >> +			totalpages += end_pfn - start_pfn;
> >> +
> >> +		size = PAGE_SIZE * (hugetlb_cma_percent * 100 * totalpages) /
> >> +			10000UL;
> >> +	} else {
> >> +		size = hugetlb_cma_size;
> >> +	}
> >> +
> >> +	pr_info("hugetlb_cma: reserve %llu, %llu per node\n", size,
> >> +		size / nr_online_nodes);
> >> +
> >> +	size /= nr_online_nodes;
> >> +
> >> +	for_each_node_state(nid, N_ONLINE) {
> >> +		unsigned long min_pfn = 0, max_pfn = 0;
> >> +
> >> +		for_each_mem_pfn_range(i, nid, &start_pfn, &end_pfn, NULL) {
> >> +			if (!min_pfn)
> >> +				min_pfn = start_pfn;
> >> +			max_pfn = end_pfn;
> >> +		}
> > 
> > Do you want to compare the range to the size? But besides that, I
> > believe this really needs to be much more careful. I believe you do not
> > want to eat a considerable part of the kernel memory because the
> > resulting configuration will really struggle (yeah all the low mem/high
> > mem problems all over again).
> 
> Will it struggle any worse than if the we allocated the same amount of memory
> for gigantic pages as is done today?  Of course, sys admins may think reserving
> memory for CMA is better than pre-allocating and end up reserving a greater
> amount.

Yes the later is my main concern. It requires to have a deep MM
understanding to realize what the lowmem problem is. Even though who
might be familiar consider it 32b relict of the past. I have seen that
several times wrt. unproportional ZONE_MOVABLE sizing already.

-- 
Michal Hocko
SUSE Labs