linux-kernel - Re: [PATCH v17 4/6] virtio-balloon: VIRTIO_BALLOON_F

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Message-Id: <201711042028.EGB64074.FOLMHtFJVQOOFS@I-love.SAKURA.ne.jp>
Date:   Sat, 4 Nov 2017 20:28:47 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     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
Cc:     david@...hat.com, cornelia.huck@...ibm.com,
        mgorman@...hsingularity.net, aarcange@...hat.com,
        amit.shah@...hat.com, pbonzini@...hat.com, willy@...radead.org,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu@...yun.com
Subject: Re: [PATCH v17 4/6] virtio-balloon: VIRTIO_BALLOON_F_SG

Wei Wang wrote:
> On 11/03/2017 07:25 PM, Tetsuo Handa wrote:
> >> @@ -184,8 +307,12 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
> >>   
> >>   	num_allocated_pages = vb->num_pfns;
> >>   	/* Did we get any? */
> >> -	if (vb->num_pfns != 0)
> >> -		tell_host(vb, vb->inflate_vq);
> >> +	if (vb->num_pfns) {
> >> +		if (use_sg)
> >> +			tell_host_sgs(vb, vb->inflate_vq, pfn_min, pfn_max);
> > Please describe why tell_host_sgs() can work without __GFP_DIRECT_RECLAIM allocation,
> > for tell_host_sgs() is called with vb->balloon_lock mutex held.
> 
> Essentially, 
> tell_host_sgs()-->send_balloon_page_sg()-->add_one_sg()-->virtqueue_add_inbuf( 
> , , num=1 ,,GFP_KERNEL)
> won't need any memory allocation, because we always add one sg (i.e. 
> num=1) each time. That memory
> allocation option is only used when multiple sgs are added (i.e. num > 
> 1) and the implementation inside virtqueue_add_inbuf
> need allocation of indirect descriptor table.
> 
> We could also add some comments above the function to explain a little 
> about this if necessary.

Yes, please do so.

Or maybe replace GFP_KERNEL with GFP_NOWAIT or 0. Though Michael might remove that GFP
argument ( http://lkml.kernel.org/r/201710022344.JII17368.HQtLOMJOOSFFVF@I-love.SAKURA.ne.jp ).

> > If this is inside vb->balloon_lock mutex (isn't this?), xb_set_page() must not
> > use __GFP_DIRECT_RECLAIM allocation, for leak_balloon_sg_oom() will be blocked
> > on vb->balloon_lock mutex.
> 
> OK. Since the preload() doesn't need too much memory (< 4K in total), 
> how about GFP_NOWAIT here?

Maybe GFP_NOWAIT | __GFP_NOWARN ?