Re: [PATCH] mm/hugetlb_cma: restrict hugetlb_cma parameter to gigantic-page alignment

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

> On May 3, 2026, at 16:42, Sang-Heon Jeon <ekffu200098@gmail.com> wrote:
> 
> Existing hugetlb_cma parameter handling logic rejects sizes smaller than
> one gigantic page, but rounds up larger sizes that are not a multiple
> of it. The two behaviors are inconsistent and neither is documented.
> 
> To remove existing inconsistent and undefined behavior, restrict
> hugetlb_cma parameter to only accept multiples of the gigantic page size.
> 
> After this restriction, the redundant round_up() in the allocation loop
> can be removed.
> 
> The new restriction is also documented in kernel-parameters.txt.
> 
> Also, including other minor changes for readability improvement with no
> functional change.
> 
> Suggested-by: Muchun Song <muchun.song@linux.dev>
> Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
> ---
> QEMU based test result
> 
> 1) hugetlb_cma=1300M
>  a) AS-IS : total 2G reserved
> [    0.000000] hugetlb_cma: reserve 1300 MiB, up to 1024 MiB per node
> [    0.000000] cma: Reserved 1024 MiB in 1 range
> [    0.000000] hugetlb_cma: reserved 1024 MiB on node 0
> [    0.000000] cma: Reserved 1024 MiB in 1 range
> [    0.000000] hugetlb_cma: reserved 1024 MiB on node 1
> 
>  b) TO-BE : rejected
> [    0.000000] hugetlb_cma: cma area must be a multiple of 1024 MiB
> 
> 2) hugetlb_cma=0:1300M,1:1G
>  a) AS-IS : 2G reserved on node 0, 1G reserved on node 1
> [    0.000000] cma: Reserved 2048 MiB in 1 range
> [    0.000000] hugetlb_cma: reserved 2048 MiB on node 0
> [    0.000000] cma: Reserved 1024 MiB in 1 range
> [    0.000000] hugetlb_cma: reserved 1024 MiB on node 1
> 
>  b) TO-BE : 1G reserved on node 1
> [    0.000000] hugetlb_cma: cma area of node 0 must be a multiple of 1024 MiB
> [    0.000000] cma: Reserved 1024 MiB in 1 range
> [    0.000000] hugetlb_cma: reserved 1024 MiB on node 1
> ---
> Hello,
> 
> This patch implements a new restriction to hugetlb_cma parameter that
> was suggested during the review of the previous patch [1].
> Thanks to Muchun for the guidance.
> 
> [1] https://lore.kernel.org/all/BFB1F124-B599-4832-A9DA-F4931999BAF9@linux.dev/
> 
> Best Regards,
> Sang-Heon Jeon
> ---
> .../admin-guide/kernel-parameters.txt         |  4 +++
> mm/hugetlb_cma.c                              | 35 +++++++++----------
> 2 files changed, 20 insertions(+), 19 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 4d0f545fb3ec..23be2f64439c 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2100,6 +2100,10 @@ Kernel parameters
> 			Format: nn[KMGTPE] or (node format)
> 			<node>:nn[KMGTPE][,<node>:nn[KMGTPE]]
> 
> + 			The size must be a multiple of the gigantic page size.
> + 			When using node format, this applies to each per-node size.
> + 			Missaligned values are dropped with a warning.
                            ^
			 Misaligned

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

Thanks.