Re: [PATCH V2] mm/page_alloc: Add alloc_contig_pages()

[Michal Hocko]

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