linux-kernel - Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address

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Date:   Thu, 7 Mar 2019 10:34:39 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterx@...hat.com, linux-mm@...ck.org, aarcange@...hat.com
Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel
 virtual address

On Thu, Mar 07, 2019 at 10:45:57AM +0800, Jason Wang wrote:
> 
> On 2019/3/7 上午12:31, Michael S. Tsirkin wrote:
> > > +static void vhost_set_vmap_dirty(struct vhost_vmap *used)
> > > +{
> > > +	int i;
> > > +
> > > +	for (i = 0; i < used->npages; i++)
> > > +		set_page_dirty_lock(used->pages[i]);
> > This seems to rely on page lock to mark page dirty.
> > 
> > Could it happen that page writeback will check the
> > page, find it clean, and then you mark it dirty and then
> > invalidate callback is called?
> > 
> > 
> 
> Yes. But does this break anything?
> The page is still there, we just remove a
> kernel mapping to it.
> 
> Thanks

Yes it's the same problem as e.g. RDMA:
	we've just marked the page as dirty without having buffers.
	Eventually writeback will find it and filesystem will complain...
	So if the pages are backed by a non-RAM-based filesystem, it’s all just broken.

one can hope that RDMA guys will fix it in some way eventually.
For now, maybe add a flag in e.g. VMA that says that there's no
writeback so it's safe to mark page dirty at any point?





-- 
MST