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

[Doug Berger]

On 06/28/2017 11:23 PM, Gregory Fong wrote:
> On Wed, Jun 28, 2017 at 10:07 AM, Doug Berger <opendmb@xxxxxxxxx> 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.
Thanks go to Angus for that!

>> 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.
It would be difficult to explain in a general way since the end user is
requesting the alignment and only she knows what the consequences would
be for insufficient alignment.

I assume in general with the CMA it is most likely a DMA constraint.
However, in our particular case the problem affected an allocation used
by a co-processor.  The larger CONFIG_CMA_ALIGNMENT is the less likely
users would run into this bug.  We encountered it after reducing our
default CONFIG_CMA_ALIGNMENT.

I agree that it should be considered for 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@xxxxxxxxxxxxxx>
>> Signed-off-by: Doug Berger <opendmb@xxxxxxxxx>
> 
> Acked-by: Gregory Fong <gregory.0xf0@xxxxxxxxx>
>