linux-kernel - Re: [PATCH] mm/hugetlb: align cma on allocation order, not demotion order

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Message-ID: <f44e3b1b-d8f7-4b5c-a66b-13ae3f3d53bd@redhat.com>
Date: Thu, 2 May 2024 15:15:53 +0200
From: David Hildenbrand <david@...hat.com>
To: Frank van der Linden <fvdl@...gle.com>, linux-mm@...ck.org,
 muchun.song@...ux.dev, akpm@...ux-foundation.org
Cc: linux-kernel@...r.kernel.org, Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [PATCH] mm/hugetlb: align cma on allocation order, not demotion
 order

On 30.04.24 18:14, Frank van der Linden wrote:
> Align the CMA area for hugetlb gigantic pages to their size, not the
> size that they can be demoted to. Otherwise there might be misaligned
> sections at the start and end of the CMA area that will never be used
> for hugetlb page allocations.
> 
> Signed-off-by: Frank van der Linden <fvdl@...gle.com>
> Cc: Roman Gushchin <roman.gushchin@...ux.dev>
> Fixes: a01f43901cfb ("hugetlb: be sure to free demoted CMA pages to CMA")
> ---
>   mm/hugetlb.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 5dc3f5ea3a2e..cfe7b025c576 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -7794,7 +7794,7 @@ void __init hugetlb_cma_reserve(int order)
>   		 * huge page demotion.
>   		 */
>   		res = cma_declare_contiguous_nid(0, size, 0,
> -					PAGE_SIZE << HUGETLB_PAGE_ORDER,
> +					PAGE_SIZE << order,
>   					HUGETLB_PAGE_ORDER, false, name,
>   					&hugetlb_cma[nid], nid);
>   		if (res) {

I was wondering how that worked when reviewing your other patch. 
Wondering why we never got a BUG report, maybe we were always lucky 
about the alignment we actually got?

We round up size to PAGE_SIZE << order, so that's the alignment we need.

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb