On Tue, Oct 11, 2016 at 09:24:50PM +0800, zijun_hu wrote: > From: zijun_hu <zijun_hu@xxxxxxx> > > the LSB of a chunk->map element is used for free/in-use flag of a area > and the other bits for offset, the sufficient and necessary condition of > this usage is that both size and alignment of a area must be even numbers > however, pcpu_alloc() doesn't force its @align parameter a even number > explicitly, so a odd @align maybe causes a series of errors, see below > example for concrete descriptions. > > lets 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 > > as explained above, inaccurate either offset or free/in-use state of > a area is recorded into relevant chunk->map element if request a odd > alignment area, and so causes memory leakage issue > > fix it by forcing the @align of a area to allocate a even number > as do for @size. > > BTW, macro ALIGN() within pcpu_fit_in_area() is replaced by roundup() too > due to back reason. in order to align a value @v up to @a boundary, macro > roundup(v, a) is more generic than ALIGN(x, a); the latter doesn't work > well when @a isn't a power of 2 value. for example, roundup(10, 6) == 12 > but ALIGN(10, 6) == 10, the former result is desired obviously > > Signed-off-by: zijun_hu <zijun_hu@xxxxxxx> Nacked-by: Tejun Heo <tj@xxxxxxxxxx> This is a fix for an imaginary problem. The most we should do about odd alignment is triggering a WARN_ON. Thanks. -- tejun -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>