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

[Cho KyongHo <pullip.cho@samsung.com>]

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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.

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