Re: [PATCH] cma: fix calculation of aligned offset

[Gregory Fong <gregory.0xf0@gmail.com>]

On Wed, Jun 28, 2017 at 10:07 AM, Doug Berger <opendmb@gmail.com> wrote:
> The align_offset parameter is used by bitmap_find_next_zero_area_off()
> to represent the offset of map's base from the previous alignment
> boundary; the function ensures that the returned index, plus the
> align_offset, honors the specified align_mask.
>
> The logic introduced by commit b5be83e308f7 ("mm: cma: align to
> physical address, not CMA region position") has the cma driver
> calculate the offset to the *next* alignment boundary.

Wow, I had that completely backward, nice catch.

> In most cases,
> the base alignment is greater than that specified when making
> allocations, resulting in a zero offset whether we align up or down.
> In the example given with the commit, the base alignment (8MB) was
> half the requested alignment (16MB) so the math also happened to work
> since the offset is 8MB in both directions.  However, when requesting
> allocations with an alignment greater than twice that of the base,
> the returned index would not be correctly aligned.

It may be worth explaining what impact incorrect alignment has for an
end user, then considering for inclusion in stable.

>
> Also, the align_order arguments of cma_bitmap_aligned_mask() and
> cma_bitmap_aligned_offset() should not be negative so the argument
> type was made unsigned.
>
> Fixes: b5be83e308f7 ("mm: cma: align to physical address, not CMA region position")
> Signed-off-by: Angus Clark <angus@angusclark.org>
> Signed-off-by: Doug Berger <opendmb@gmail.com>

Acked-by: Gregory Fong <gregory.0xf0@gmail.com>