Re: [PATCH] mm/cma.c: use exact_nid true to fix possible per-numa cma leak

[Roman Gushchin <guro@fb.com>]

On Sun, Jun 28, 2020 at 07:43:45PM +1200, Barry Song wrote:
> Calling cma_declare_contiguous_nid() with false exact_nid for per-numa
> reservation can easily cause cma leak and various confusion.
> For example, mm/hugetlb.c is trying to reserve per-numa cma for gigantic
> pages. But it can easily leak cma and make users confused when system has
> memoryless nodes.
> 
> In case the system has 4 numa nodes, and only numa node0 has memory.
> if we set hugetlb_cma=4G in bootargs, mm/hugetlb.c will get 4 cma areas
> for 4 different numa nodes. since exact_nid=false in current code, all
> 4 numa nodes will get cma successfully from node0, but hugetlb_cma[1 to 3]
> will never be available to hugepage will only allocate memory from
> hugetlb_cma[0].
> 
> In case the system has 4 numa nodes, both numa node0&2 has memory, other
> nodes have no memory.
> if we set hugetlb_cma=4G in bootargs, mm/hugetlb.c will get 4 cma areas
> for 4 different numa nodes. since exact_nid=false in current code, all
> 4 numa nodes will get cma successfully from node0 or 2, but hugetlb_cma[1]
> and [3] will never be available to hugepage as mm/hugetlb.c will only
> allocate memory from hugetlb_cma[0] and hugetlb_cma[2].
> This causes permanent leak of the cma areas which are supposed to be
> used by memoryless node.
> 
> Of cource we can workaround the issue by letting mm/hugetlb.c scan all
> cma areas in alloc_gigantic_page() even node_mask includes node0 only.
> that means when node_mask includes node0 only, we can get page from
> hugetlb_cma[1] to hugetlb_cma[3]. But this will cause kernel crash in
> free_gigantic_page() while it wants to free page by:
> cma_release(hugetlb_cma[page_to_nid(page)], page, 1 << order)
> 
> On the other hand, exact_nid=false won't consider numa distance, it
> might be not that useful to leverage cma areas on remote nodes.
> I feel it is much simpler to make exact_nid true to make everything
> clear. After that, memoryless nodes won't be able to reserve per-numa
> CMA from other nodes which have memory.

Totally agree.

Acked-by: Roman Gushchin <guro@fb.com>

Thanks!

> Fixes: cf11e85fc08c ("mm: hugetlb: optionally allocate gigantic hugepages using cma")
> Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Cc: Aslan Bakirov <aslan@fb.com>
> Cc: Roman Gushchin <guro@fb.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Andreas Schaufler <andreas.schaufler@gmx.de>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Rik van Riel <riel@surriel.com>
> Cc: Joonsoo Kim <js1304@gmail.com>
> Cc: Robin Murphy <robin.murphy@arm.com>
> Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
> ---
>  mm/cma.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/cma.c b/mm/cma.c
> index b24151fa2101..f472f398026f 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -338,13 +338,13 @@ int __init cma_declare_contiguous_nid(phys_addr_t base,
>  		 */
>  		if (base < highmem_start && limit > highmem_start) {
>  			addr = memblock_alloc_range_nid(size, alignment,
> -					highmem_start, limit, nid, false);
> +					highmem_start, limit, nid, true);
>  			limit = highmem_start;
>  		}
>  
>  		if (!addr) {
>  			addr = memblock_alloc_range_nid(size, alignment, base,
> -					limit, nid, false);
> +					limit, nid, true);
>  			if (!addr) {
>  				ret = -ENOMEM;
>  				goto err;
> -- 
> 2.27.0
> 
>