linux-kernel - RE: [RFC 00/10] implement alternative and much simpler id allocator

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Date:   Sun, 18 Dec 2016 02:42:07 +0000
From:   Matthew Wilcox <mawilcox@...rosoft.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
CC:     Tejun Heo <tj@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Jens Axboe" <axboe@...nel.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>
Subject: RE: [RFC 00/10] implement alternative and much simpler id allocator

From: Matthew Wilcox
> From: Matthew Wilcox
> > Heh, I was thinking about that too.  The radix tree supports "exceptional
> > entries" which have the bottom bit set.  On a 64-bit machine, we could use
> 62
> > of the bits in the radix tree root to store the ID bitmap.  I'm a little wary of
> the
> > potential complexity, but we should try it out.
> 
> Test patch here: http://git.infradead.org/users/willy/linux-
> dax.git/shortlog/refs/heads/idr-2016-12-16
> It passes the test suite ... which I actually had to adjust because it now succeeds
> in cases where it hadn't (allocating ID 0 without preallocating), and it will now
> fail in cases where it hadn't previously (assuming a single preallocation would
> be enough).  There shouldn't be any examples of that in the kernel proper; it
> was simply me being lazy when I wrote the test suite.

Found a bug, committed a fix (and another test case).  It now no longer returns -EAGAIN in situations where it wouldn't have, so I've reverted that bit of the test suite change.  Still succeeds when it wouldn't have before, but I feel no pressure to change that ;-)