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

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Date:   Thu, 30 Nov 2017 06:39:52 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     wei.w.wang@...el.com, 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, mhocko@...nel.org,
        akpm@...ux-foundation.org, mawilcox@...rosoft.com,
        david@...hat.com, cornelia.huck@...ibm.com,
        mgorman@...hsingularity.net, aarcange@...hat.com,
        amit.shah@...hat.com, pbonzini@...hat.com,
        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

On Thu, Nov 30, 2017 at 10:35:03PM +0900, Tetsuo Handa wrote:
> 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)?

It happens all the time.  The vast majority of users of the IDA set
low bits.  Also, it's the first 62 bits -- going up to 63 bits with the
XArray rewrite.

I do plan to redo the xbitmap on top of the XArray; I'm just trying to
get the XArray merged first.  The IDA and xbitmap code will share much
more code when that happens.