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 13:45:40 -0700
From: Frank van der Linden <fvdl@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, muchun.song@...ux.dev, linux-kernel@...r.kernel.org, 
	Marek Szyprowski <m.szyprowski@...sung.com>, David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 1/2] mm/cma: drop incorrect alignment check in cma_init_reserved_mem

On Thu, Apr 4, 2024 at 1:15 PM Andrew Morton <akpm@...ux-foundationorg> wrote:
>
> On Thu,  4 Apr 2024 16:25:14 +0000 Frank van der Linden <fvdl@...gle.com> 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).
> >
> > 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.
>
> What are the userspace visible effects of this bug?

None that I know of. This bug was exposed because I made the hugetlb
code correctly pass the right order_per_bit argument (see the
accompanying hugetlb cma fix), which then tripped this check when I
backported it to an older kernel, passing an order of 30 (1G hugetlb
page) as order_per_bit. This actually won't happen for 6.9-rc, since
the (intended) order_per_bit was reduced to HUGETLB_PAGE_ORDER because
of hugetlb page demotion.

So, no user visible effects. However, if the other fix is going to be
backported, this one is a prereq.

- Frank