On Thu 17-10-19 09:21:24, David Hildenbrand wrote: > On 17.10.19 09:11, Michal Hocko wrote: > > On Thu 17-10-19 10:44:41, Anshuman Khandual wrote: > > [...] > > > Does this add-on documentation look okay ? Should we also mention about > > > the > > > possible reduction in chances of success during pfn block search for the > > > non-power-of-two cases as the implicit alignment will probably turn out to > > > be bigger than nr_pages itself ? > > > > > > * Requested nr_pages may or may not be power of two. The search for > > > suitable > > > * memory range in a zone happens in nr_pages aligned pfn blocks. But in > > > case > > > * when nr_pages is not power of two, an implicitly aligned pfn block > > > search > > > * will happen which in turn will impact allocated memory block's > > > alignment. > > > * In these cases, the size (i.e nr_pages) and the alignment of the > > > allocated > > > * memory will be different. This problem does not exist when nr_pages > > > is power > > > * of two where the size and the alignment of the allocated memory will > > > always > > > * be nr_pages. > > > > I dunno, it sounds more complicated than really necessary IMHO. Callers > > shouldn't really be bothered by memory blocks and other really deep > > implementation details.. Wouldn't be the below sufficient? > > > > The allocated memory is always aligned to a page boundary. If nr_pages > > is a power of two then the alignement is guaranteed to be to the given > > s/alignement/alignment/ > > and "the PFN is guaranteed to be aligned to nr_pages" (the address is > aligned to nr_pages*PAGE_SIZE)
thx for the correction. > > nr_pages (e.g. 1GB request would be aligned to 1GB). > > > > I'd probably add "This function will miss allocation opportunities if > nr_pages is not a power of two (and the implicit alignment is bogus)." This is again an implementation detail and quite a confusing one to whoever not familiar with the MM internals. And to be fair even a proper alignment doesn't give you any stronger guarantee as long as the allocation operates on non movable zones anyway. -- Michal Hocko SUSE Labs
