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
