Re: [PATCH] mm/hugetlb_cma: round up per_node before logging it

[Muchun Song <muchun.song@linux.dev>]

> On Apr 22, 2026, at 22:33, Sang-Heon Jeon <ekffu200098@gmail.com> wrote:
> 
> When the user requests a total hugetlb CMA size without per-node
> specification, hugetlb_cma_reserve() computes per_node from
> hugetlb_cma_size and the number of nodes that have memory
> 
>        per_node = DIV_ROUND_UP(hugetlb_cma_size,
>                                nodes_weight(hugetlb_bootmem_nodes));
> 
> The reservation loop later computes
> 
>        size = round_up(min(per_node, hugetlb_cma_size - reserved),
>                          PAGE_SIZE << order);
> 
> So the actually reserved per_node size is multiple of (PAGE_SIZE <<
> order), but the logged per_node is not rounded up, so it may be smaller
> than the actual reserved size.
> 
> For example, as the existing comment describes, if a 3 GB area is
> requested on a machine with 4 NUMA nodes that have memory, 1 GB is
> allocated on the first three nodes, but the printed log is
> 
>        hugetlb_cma: reserve 3072 MiB, up to 768 MiB per node
> 
> Round per_node up to (PAGE_SIZE << order) before logging so that the
> printed log always matches the actual reserved size. No functional change
> to the actual reservation size, as the following case analysis shows
> 
> 1. remaining (hugetlb_cma_size - reserved) >= rounded per_node
> - AS-IS: min() picks unrounded per_node;
>    round_up() returns rounded per_node
> - TO-BE: min() picks rounded per_node;
>    round_up() returns rounded per_node (no-op)
> 2. remaining < unrounded per_node
> - AS-IS: min() picks remaining;
>    round_up() returns round_up(remaining)
> - TO-BE: min() picks remaining;
>    round_up() returns round_up(remaining)
> 3. unrounded per_node <= remaining < rounded per_node
> - AS-IS: min() picks unrounded per_node;
>    round_up() returns rounded per_node
> - TO-BE: min() picks remaining;
>    round_up() returns round_up(remaining) equals rounded per_node
> 
> Fixes: cf11e85fc08c ("mm: hugetlb: optionally allocate gigantic hugepages using cma") # 5.7
                                                                                         ^
                                                                           we do not add a suffix to Fixes tag.
                                                                           We usually add it to stable tag.

Reviewed-by: Muchun Song <muchun.song@linux.dev>

Thanks.