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

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Message-ID: <CAPTztWaMq7ogS_vgBTzUSmoosb1XtNO7FoYSSsUz-QEJBZtCLg@mail.gmail.com>
Date: Thu, 4 Apr 2024 13:48:45 -0700
From: Frank van der Linden <fvdl@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, muchun.song@...ux.dev, akpm@...ux-foundation.org, 
	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 Thu, Apr 4, 2024 at 1:05 PM David Hildenbrand <david@...hat.com> wrote:
>
> 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.

Nobody stumbled over it because the only user of CMA that should have
passed in an order_per_bit large enough to trigger this was
hugetlb_cma. However, because of a bug, it didn't :) When I fixed
that, I noticed that this check fired.

- Frank