linux-kernel - Re: [PATCH v2 05/14] mm, memremap: Up-level foreach_order

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Date:   Mon, 16 Jul 2018 14:00:14 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     akpm@...ux-foundation.org, Logan Gunthorpe <logang@...tatee.com>,
        vishal.l.verma@...el.com, hch@....de, linux-mm@...ck.org,
        jack@...e.cz, linux-nvdimm@...ts.01.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 05/14] mm, memremap: Up-level foreach_order_pgoff()

On Mon, Jul 16, 2018 at 10:00:48AM -0700, Dan Williams wrote:
> The foreach_order_pgoff() helper takes advantage of the ability to
> insert multi-order entries into a radix. It is currently used by
> devm_memremap_pages() to minimize the number of entries in the pgmap
> radix. Instead of dividing a range by a constant power-of-2 sized unit
> and inserting an entry for each unit, it determines the maximum
> power-of-2 sized entry (subject to alignment offset) that can be
> inserted at each iteration.
> 
> Up-level this helper so it can be used for populating other radix
> instances. For example asynchronous-memmap-initialization-thread lookups
> arriving in a follow on change.

Hopefully by the time you're back, I'll have this code replaced with
the XArray.  Here's my proposed API:

	old = xa_store_range(xa, first, last, ptr, GFP_KERNEL);

and then you'd simply use xa_for_each() as an iterator.  You'd do one
iteration for each range in the XArray, not for each entry occupied.
So there's a difference between:

	xa_store(xa, 1, ptr, GFP_KERNEL);
	xa_store(xa, 2, ptr, GFP_KERNEL);
	xa_store(xa, 3, ptr, GFP_KERNEL);

and

	xa_store_range(xa, 1, 3, ptr, GFP_KERNEL);

	index = 0; i = 0;
	xa_for_each(xa, p, index, ULONG_MAX, XA_PRESENT)
		i++;

will return i = 3 for the first case and i = 1 for the second.