linux-kernel - Re: [PATCH v19 3/7] xbitmap: add more operations

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Message-Id: <201712162028.FEB87079.FOJFMQHVOSLtFO@I-love.SAKURA.ne.jp>
Date:   Sat, 16 Dec 2017 20:28:04 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     wei.w.wang@...el.com, willy@...radead.org
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,
        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 v19 3/7] xbitmap: add more operations

Wei Wang wrote:
> On 12/16/2017 02:42 AM, Matthew Wilcox wrote:
> > On Tue, Dec 12, 2017 at 07:55:55PM +0800, Wei Wang wrote:
> >> +int xb_preload_and_set_bit(struct xb *xb, unsigned long bit, gfp_t gfp);
> > I'm struggling to understand when one would use this.  The xb_ API
> > requires you to handle your own locking.  But specifying GFP flags
> > here implies you can sleep.  So ... um ... there's no locking?
> 
> In the regular use cases, people would do xb_preload() before taking the 
> lock, and the xb_set/clear within the lock.
> 
> In the virtio-balloon usage, we have a large number of bits to set with 
> the balloon_lock being held (we're not unlocking for each bit), so we 
> used the above wrapper to do preload and set within the balloon_lock, 
> and passed in GFP_NOWAIT to avoid sleeping. Probably we can change to 
> put this wrapper implementation to virtio-balloon, since it would not be 
> useful for the regular cases.

GFP_NOWAIT is chosen in order not to try to OOM-kill something, isn't it?
But passing GFP_NOWAIT means that we can handle allocation failure. There is
no need to use preload approach when we can handle allocation failure.