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 13:52:55 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: David Hildenbrand <david@...hat.com>
Cc: Frank van der Linden <fvdl@...gle.com>, 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 04, 2024 at 10:13:21PM +0200, David Hildenbrand wrote:
> 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 :)

We probably should not rush with a stable backporting, especially given your
next comment on page sizes on arm.