netdev - Re: [PATCH V5 0/9] Fixes for vhost metadata acceleration

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Date:   Mon, 12 Aug 2019 10:02:52 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH V5 0/9] Fixes for vhost metadata acceleration

On Mon, Aug 12, 2019 at 05:49:08AM -0400, Michael S. Tsirkin wrote:
> On Mon, Aug 12, 2019 at 10:44:51AM +0800, Jason Wang wrote:
> > 
> > On 2019/8/11 上午1:52, Michael S. Tsirkin wrote:
> > > On Fri, Aug 09, 2019 at 01:48:42AM -0400, Jason Wang wrote:
> > > > Hi all:
> > > > 
> > > > This series try to fix several issues introduced by meta data
> > > > accelreation series. Please review.
> > > > 
> > > > Changes from V4:
> > > > - switch to use spinlock synchronize MMU notifier with accessors
> > > > 
> > > > Changes from V3:
> > > > - remove the unnecessary patch
> > > > 
> > > > Changes from V2:
> > > > - use seqlck helper to synchronize MMU notifier with vhost worker
> > > > 
> > > > Changes from V1:
> > > > - try not use RCU to syncrhonize MMU notifier with vhost worker
> > > > - set dirty pages after no readers
> > > > - return -EAGAIN only when we find the range is overlapped with
> > > >    metadata
> > > > 
> > > > Jason Wang (9):
> > > >    vhost: don't set uaddr for invalid address
> > > >    vhost: validate MMU notifier registration
> > > >    vhost: fix vhost map leak
> > > >    vhost: reset invalidate_count in vhost_set_vring_num_addr()
> > > >    vhost: mark dirty pages during map uninit
> > > >    vhost: don't do synchronize_rcu() in vhost_uninit_vq_maps()
> > > >    vhost: do not use RCU to synchronize MMU notifier with worker
> > > >    vhost: correctly set dirty pages in MMU notifiers callback
> > > >    vhost: do not return -EAGAIN for non blocking invalidation too early
> > > > 
> > > >   drivers/vhost/vhost.c | 202 +++++++++++++++++++++++++-----------------
> > > >   drivers/vhost/vhost.h |   6 +-
> > > >   2 files changed, 122 insertions(+), 86 deletions(-)
> > > This generally looks more solid.
> > > 
> > > But this amounts to a significant overhaul of the code.
> > > 
> > > At this point how about we revert 7f466032dc9e5a61217f22ea34b2df932786bbfc
> > > for this release, and then re-apply a corrected version
> > > for the next one?
> > 
> > 
> > If possible, consider we've actually disabled the feature. How about just
> > queued those patches for next release?
> > 
> > Thanks
> 
> Sorry if I was unclear. My idea is that
> 1. I revert the disabled code
> 2. You send a patch readding it with all the fixes squashed
> 3. Maybe optimizations on top right away?
> 4. We queue *that* for next and see what happens.
> 
> And the advantage over the patchy approach is that the current patches
> are hard to review. E.g.  it's not reasonable to ask RCU guys to review
> the whole of vhost for RCU usage but it's much more reasonable to ask
> about a specific patch.

I think there are other problems here too, I don't like that the use
of mmu notifiers is so different from every other driver, or that GUP
is called under spinlock.

So I favor the revert and try again approach as well. It is hard to
get a clear picture with these endless bug fix patches

Jason