On 2016/10/12 1:22, Michal Hocko wrote: > On Tue 11-10-16 21:24:50, zijun_hu wrote: >> From: zijun_hu <zijun...@htc.com> >> >> 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. > > Is or was there any user who would use a different than even (or power of 2) > alighment? If not is this really worth handling? >
it seems only a power of 2 alignment except 1 can make sure it work very well, that is a strict limit, maybe this more strict limit should be checked i don't know since there are too many sources and too many users and too many use cases. even if nobody, i can't be sure that it doesn't happens in the future it is worth since below reasons 1) if it is used in right ways, this patch have no impact; otherwise, it can alert user by warning message and correct the behavior. is it better that a warning message and correcting than resulting in many terrible error silently under a special case by change? it can make program more stronger. 2) does any alignment but 1 means a power of 2 alignment conventionally and implicitly? if not, is it better that adjusting both @align and @size uniformly based on the sufficient necessary condition than mixing supposing one part is right and correcting the other? i find that there is BUG_ON(!is_power_of_2(align)) statement in mm/vmalloc.c 3) this simple fix can make the function applicable in wider range, it hints the reader that the lowest requirement for alignment is a even number 4) for char a[10][10]; char (*p)[10]; if a user want to allocate a @size = 10 and @align = 10 memory block, should we reject the user's request?
