percpu: ensure the requested alignment is power of two
The percpu allocator expectedly assumes that the requested alignment
is power of two but hasn't been veryfing the input. If the specified
alignment isn't power of two, the allocator can malfunction. Add the
sanity check.
The following is detailed analysis of the effects of alignments which
aren't power of two.
The alignment must be a even at least since the LSB of a chunk->map
element is used as free/in-use flag of a area; besides, the alignment
must be a power of 2 too since ALIGN() doesn't work well for other
alignment always but is adopted by pcpu_fit_in_area(). IOW, the
current allocator only works well for a power of 2 aligned area
allocation.
See below opposite example for why an odd alignment doesn't work.
Let's assume area [16, 36) is free but its previous one is in-use, we
want to allocate a @size == 8 and @align == 7 area. The larger area
[16, 36) is split to three areas [16, 21), [21, 29), [29, 36)
eventually. However, due to the usage for a chunk->map element, the
actual offset of the aim area [21, 29) is 21 but is recorded in
relevant element as 20; moreover, the residual tail free area [29,
36) is mistook as in-use and is lost silently
Unlike macro roundup(), ALIGN(x, a) doesn't work if @a isn't a power
of 2 for example, roundup(10, 6) == 12 but ALIGN(10, 6) == 10, and
the latter result isn't desired obviously.
tj: Code style and patch description updates.
Signed-off-by: zijun_hu <zijun_hu@htc.com>
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
2 files changed