On 10/16/2019 06:11 PM, Michal Hocko wrote: > On Wed 16-10-19 14:29:05, David Hildenbrand wrote: >> On 16.10.19 13:51, Michal Hocko wrote: >>> On Wed 16-10-19 16:43:57, Anshuman Khandual wrote: >>>> >>>> >>>> On 10/16/2019 04:39 PM, David Hildenbrand wrote: >>> [...] >>>>> Just to make sure, you ignored my comment regarding alignment >>>>> although I explicitly mentioned it a second time? Thanks. >>>> >>>> I had asked Michal explicitly what to be included for the respin. Anyways >>>> seems like the previous thread is active again. I am happy to incorporate >>>> anything new getting agreed on there. >>> >>> Your patch is using the same alignment as the original code would do. If >>> an explicit alignement is needed then this can be added on top, right? >>> >> >> Again, the "issue" I see here is that we could now pass in numbers that are >> not a power of two. For gigantic pages it was clear that we always have a >> number of two. The alignment does not make any sense otherwise. ALIGN() does expect nr_pages two be power of two otherwise the mask value might not be correct, affecting start pfn value for a zone. #define ALIGN(x, a) __ALIGN_KERNEL((x), (a)) #define __ALIGN_KERNEL(x, a) __ALIGN_KERNEL_MASK(x, (typeof(x))(a) - 1) #define __ALIGN_KERNEL_MASK(x, mask) (((x) + (mask)) & ~(mask)) >> >> What I'm asking for is >> >> a) Document "The resulting PFN is aligned to nr_pages" and "nr_pages should >> be a power of two". > > OK, this makes sense. Sure, will add this to the alloc_contig_pages() helper description and in the commit message as well. > >> b) Eventually adding something like >> >> if (WARN_ON_ONCE(!is_power_of_2(nr_pages))) >> return NULL; > > I am not sure this is really needed. > Just wondering why not ? Ideally if we are documenting that nr_pages should be power of two, then we should abort erring allocation request with an warning. Is it because allocation can still succeed for non-power-of-two requests despite possible problem with mask and alignment ? Hence there is no need to abort it.
