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

From: Jerome Glisse <[email-masked]>
Date: Thu, 7 Mar 2019 14:09:10 -0500

On Thu, Mar 07, 2019 at 10:34:39AM -0500, Michael S. Tsirkin wrote:
> 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?

I thought this patch was only for anonymous memory ie not file back ?
If so then set dirty is mostly useless it would only be use for swap
but for this you can use an unlock version to set the page dirty.

Cheers,
Jérôme