Messages in this thread | | | From | Gregory Fong <> | Date | Wed, 28 Jun 2017 23:23:52 -0700 | Subject | Re: [PATCH] cma: fix calculation of aligned offset |
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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>
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