linux-kernel - Re: [PATCH v18 05/10] xbitmap: add more operations

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Message-Id: <201711302235.FAJ57385.OFJHOVQOFtMSFL@I-love.SAKURA.ne.jp>
Date:   Thu, 30 Nov 2017 22:35:03 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     wei.w.wang@...el.com
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
        kvm@...r.kernel.org, linux-mm@...ck.org, mst@...hat.com,
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        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu@...yun.com, nilal@...hat.com, riel@...hat.com
Subject: Re: [PATCH v18 05/10] xbitmap: add more operations

Tetsuo Handa wrote:
> > +
> > +			if (ebit >= BITS_PER_LONG)
> > +				continue;
> 
> (I don't understand how radix tree works, but generally this patchset looks fuzzy
> to me about boundary cases. Thus, I want to confirm that this is not an overlook.)
> Why is making "ebit >= BITS_PER_LONG" (e.g. start == 62) case a no-op correct?
> Aren't there bits which should have been cleared in this case?

According to xb_set_bit(), it seems to me that we are trying to avoid memory allocation
for "struct ida_bitmap" when all set bits within a 1024-bits bitmap reside in the first
61 bits.

But does such saving help? Is there characteristic bias that majority of set bits resides
in the first 61 bits, for "bit" is "unsigned long" which holds a page number (isn't it)?
If no such bias, wouldn't eliminating radix_tree_exception() case and always storing
"struct ida_bitmap" simplifies the code (and make the processing faster)?