linux-kernel - Re: Radix tree range entries

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Message-ID: <20170201222326.GA2267@bombadil.infradead.org>
Date:   Wed, 1 Feb 2017 14:23:26 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Konstantin Khlebnikov <koct9i@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Radix tree range entries

On Wed, Feb 01, 2017 at 10:48:08AM -0800, Dan Williams wrote:
> On Tue, Jan 31, 2017 at 8:59 AM, Matthew Wilcox <willy@...radead.org> wrote:
> >     We have a confirmed customer in the shape of devm_memremap_pages(), so
> >     add support for range entries.  The documentation lists caveats on their
> >     use, but none of these caveats apply to the one user.
> >
> >     Signed-off-by: Matthew Wilcox <mawilcox@...rosoft.com>
> 
> Thanks, I'll take a look. This will let me get rid of the order_at()
> hack I have in devm_memremap_pages(). I assume there's no requirement
> that 'start' be aligned to 'len' when calling
> radix_tree_insert_range()?

No alignment restrictions.  If you find something that doesn't work,
let me know.  From the test suite:

+       for (start = 0; start < 100; start++) {
+               for (len = 1; len < 100; len++) {
+                       range_check_1(&tree, start, len);
+               }
+       }

The only restrictions I have is that len must be >0 and start+len
can't wrap past ~0UL.  These seem like reasonable restrictions to me.
Maybe we have some users who want to change the index type from unsigned
long to unsigned long long, but I think 32-bit machines are dying out
fast enough that we don't care.