On Fri, Aug 14, 2020 at 01:55:58PM -0700, Minchan Kim wrote: > On Fri, Aug 14, 2020 at 06:40:20PM +0100, Matthew Wilcox wrote: > > 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. > > Kyoungho is working on it. > I am not sure how much he could share but hopefully, he could > show some. Kyoungho? We are preparing the following patch to dma-buf heap that uses alloc_pages_bulk(). The heap collects pages of identical size from alloc_pages_bulk() for the H/Ws that have restrictions in memory alignments due to the performance or the functionality. > > > > > 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. > > I wanted to let the API wok from CMA area and/or movable zone where are > always fulled with migrable pages. > If we carry on only zone flags without pfn range, it couldn't fulfil cma > area cases. > Other reason is if user see fewer pages returned, he could try subsequent > ranges to get remained ones. > > > Is it useful to return fewer pages than requested? > > It's useful because user could ask further than what they need or retry. I agree with Minchan. A CMA area is private to a device or a device driver except the shared DMA pool. Drivers first try collecting large order pages from its private CMA area. If the number of pages returned by alloc_pages_bulk() is less than nr_elem, then the driver can retry with fewer nr_elem after a while with expecting pinned pages or pages under races in the CMA area are resolved. The driver may further try call alloc_pages_bulk() on another CMA area which is available to the driver or the movable zone.
