linux-kernel - Re: [PATCH 2/2] mm/hugetlb: pass correct order_per_bit to cma_declare_contiguous

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Date: Thu, 4 Apr 2024 22:13:21 +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 2/2] mm/hugetlb: pass correct order_per_bit to
 cma_declare_contiguous_nid

On 04.04.24 18:25, Frank van der Linden wrote:
> The hugetlb_cma code passes 0 in the order_per_bit argument to
> cma_declare_contiguous_nid (the alignment, computed using the
> page order, is correctly passed in).
> 
> This causes a bit in the cma allocation bitmap to always represent
> a 4k page, making the bitmaps potentially very large, and slower.
> 
> So, correctly pass in the order instead.
> 
> Signed-off-by: Frank van der Linden <fvdl@...gle.com>
> Cc: Roman Gushchin <roman.gushchin@...ux.dev>
> Fixes: cf11e85fc08c ("mm: hugetlb: optionally allocate gigantic hugepages using cma")

It might be subopimal, but do we call it a "BUG" that needs "fixing". I 
know, controversial :)

> ---
>   mm/hugetlb.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 23ef240ba48a..6dc62d8b2a3a 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -7873,9 +7873,9 @@ void __init hugetlb_cma_reserve(int order)
>   		 * huge page demotion.
>   		 */
>   		res = cma_declare_contiguous_nid(0, size, 0,
> -						PAGE_SIZE << HUGETLB_PAGE_ORDER,
> -						 0, false, name,
> -						 &hugetlb_cma[nid], nid);
> +					PAGE_SIZE << HUGETLB_PAGE_ORDER,
> +					HUGETLB_PAGE_ORDER, false, name,
> +					&hugetlb_cma[nid], nid);
>   		if (res) {
>   			pr_warn("hugetlb_cma: reservation failed: err %d, node %d",
>   				res, nid);

.. I'm afraid this is not completely correct.

For example, on arm64, HUGETLB_PAGE_ORDER is essentially PMD_ORDER.

.. but we do support smaller hugetlb sizes than that (cont-pte hugetlb 
size is 64 KiB, not 2 MiB -- PMD -- on a 4k kernel)

-- 
Cheers,

David / dhildenb