Re: [RFC 0/7] Support high-order page bulk allocation

[Matthew Wilcox <willy@infradead.org>]

On Fri, Aug 14, 2020 at 10:31:24AM -0700, Minchan Kim wrote:
> There is a need for special HW to require bulk allocation of
> high-order pages. For example, 4800 * order-4 pages.

... but you haven't shown that user.

>   int alloc_pages_bulk(unsigned long start, unsigned long end,
>                        unsigned int migratetype, gfp_t gfp_mask,
>                        unsigned int order, unsigned int nr_elem,
>                        struct page **pages);
> 
> It will investigate the [start, end) and migrate movable pages
> out there by best effort(by upcoming patches) to make requested
> order's free pages.
> 
> The allocated pages will be returned using pages parameter.
> Return value represents how many of requested order pages we got.
> It could be less than user requested by nr_elem.

I don't understand why a user would need to know the PFNs to allocate
between.  This seems like something that's usually specified by GFP_DMA32
or similar.

Is it useful to return fewer pages than requested?