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:
quoted
On Fri, Aug 14, 2020 at 10:31:24AM -0700, Minchan Kim wrote:
quoted
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.
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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.
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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.