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 12:40:58 -0700
From: Frank van der Linden <fvdl@...gle.com>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: linux-mm@...ck.org, muchun.song@...ux.dev, akpm@...ux-foundation.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm/hugetlb: pass correct order_per_bit to cma_declare_contiguous_nid

On Thu, Apr 4, 2024 at 11:56 AM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
>
> On Thu, Apr 04, 2024 at 04:25:15PM +0000, 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")
>
> Hi Frank,
>
> there is a comment just above your changes which explains why order_per_bit is 0.
> Is this not true anymore? If so, please, fix the comment too. Please, clarify.
>
> Thanks!

Hi Roman,

I'm assuming you're referring to this comment:

/*
 * Note that 'order per bit' is based on smallest size that
 * may be returned to CMA allocator in the case of
 * huge page demotion.
 */

That comment was added in a01f43901cfb9 ("hugetlb: be sure to free
demoted CMA pages to CMA").

It talks about HUGETLB_PAGE_ORDER being the minimum order being given
back to the CMA allocator (after hugetlb demotion), therefore
order_per_bit must be HUGETLB_PAGE_ORDER. See the commit message for
that commit:

"Therefore, at region setup time we use HUGETLB_PAGE_ORDER as the
smallest possible huge page size that can be given back to CMA."

But the commit, while correctly changing the alignment, left the
order_per_bit argument at 0,  even though it clearly intended to set
it at HUGETLB_PAGE_ORDER. The confusion may have been that
cma_declare_contiguous_nid has 9 arguments, several of which can be
left at 0 meaning 'use default', so it's easy to misread.

In other words, the comment was correct, but the code was not. After
this patch, comment and code match.