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

From: Cho KyongHo <pullip.cho@samsung.com>
To: Minchan Kim <minchan@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm <linux-mm@kvack.org>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Vlastimil Babka <vbabka@suse.cz>, John Dias <joaodias@google.com>,
	Suren Baghdasaryan <surenb@google.com>,
	hyesoo.yu@samsung.com, janghyuck.kim@samsung.com
Subject: Re: [RFC 0/7] Support high-order page bulk allocation
Date: Tue, 18 Aug 2020 18:22:15 +0900	[thread overview]
Message-ID: <20200818092215.GA191752@KEI> (raw)
In-Reply-To: <20200814205558.GA2814941@google.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?
> 
Hyesoo posted a patch series that uses alloc_pages_bulk() in a dma-heap;
please take a look at:
https://lore.kernel.org/linux-mm/20200818080415.7531-1-hyesoo.yu@samsung.com/

The patch series introduces a new type of dma-heap, chunk heap which is
initialized by a device tree node. The chunk heap also needs its device
tree node should have a phandle to reserved memory node with "reusable"
property.

> > 
> > >   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.
> 

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