linux-kernel - Re: [PATCH 1/2] mm/cma: drop incorrect alignment check in cma_init_reserved

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Date: Thu, 4 Apr 2024 22:05:27 +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, Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH 1/2] mm/cma: drop incorrect alignment check in
 cma_init_reserved_mem

On 04.04.24 18:25, Frank van der Linden wrote:
> cma_init_reserved_mem uses IS_ALIGNED to check if the size
> represented by one bit in the cma allocation bitmask is
> aligned with CMA_MIN_ALIGNMENT_BYTES (pageblock size).

I recall the important part is that our area always covers full 
pageblocks (CMA_MIN_ALIGNMENT_BYTES), because we cannot have "partial 
CMA" pageblocks.

Internally, allocating from multiple pageblock should just work.

It's late in Germany, hopefully I am not missing something

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

> 
> However, this is too strict, as this will fail if
> order_per_bit > pageblock_order, which is a valid configuration.
> 
> We could check IS_ALIGNED both ways, but since both numbers are
> powers of two, no check is needed at all.
> 
> Signed-off-by: Frank van der Linden <fvdl@...gle.com>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: David Hildenbrand <david@...hat.com>
> Fixes: de9e14eebf33 ("drivers: dma-contiguous: add initialization from device tree")

Is there are real setup/BUG we are fixing? Why did we not stumble over 
that earlier?

If so, please describe that in the patch description.

-- 
Cheers,

David / dhildenb