Re: [RFC PATCH 00/11] mm/hmm: Various revisions from a locking/code review

From: Jason Gunthorpe <jgg@ziepe.ca>
To: John Hubbard <jhubbard@nvidia.com>
Cc: linux-rdma@vger.kernel.org, linux-mm@kvack.org,
	Jerome Glisse <jglisse@redhat.com>,
	Ralph Campbell <rcampbell@nvidia.com>
Subject: Re: [RFC PATCH 00/11] mm/hmm: Various revisions from a locking/code review
Date: Thu, 23 May 2019 16:37:03 -0300	[thread overview]
Message-ID: <20190523193703.GI12159@ziepe.ca> (raw)
In-Reply-To: <6ee88cde-5365-9bbc-6c4d-7459d5c3ebe2@nvidia.com>

On Thu, May 23, 2019 at 12:04:16PM -0700, John Hubbard wrote:
> On 5/23/19 8:34 AM, Jason Gunthorpe wrote:
> > From: Jason Gunthorpe <jgg@mellanox.com>
> > 
> > This patch series arised out of discussions with Jerome when looking at the
> > ODP changes, particularly informed by use after free races we have already
> > found and fixed in the ODP code (thanks to syzkaller) working with mmu
> > notifiers, and the discussion with Ralph on how to resolve the lifetime model.
> > 
> > Overall this brings in a simplified locking scheme and easy to explain
> > lifetime model:
> > 
> >   If a hmm_range is valid, then the hmm is valid, if a hmm is valid then the mm
> >   is allocated memory.
> > 
> >   If the mm needs to still be alive (ie to lock the mmap_sem, find a vma, etc)
> >   then the mmget must be obtained via mmget_not_zero().
> > 
> > Locking of mm->hmm is shifted to use the mmap_sem consistently for all
> > read/write and unlocked accesses are removed.
> > 
> > The use unlocked reads on 'hmm->dead' are also eliminated in favour of using
> > standard mmget() locking to prevent the mm from being released. Many of the
> > debugging checks of !range->hmm and !hmm->mm are dropped in favour of poison -
> > which is much clearer as to the lifetime intent.
> > 
> > The trailing patches are just some random cleanups I noticed when reviewing
> > this code.
> > 
> > I expect Jerome & Ralph will have some design notes so this is just RFC, and
> > it still needs a matching edit to nouveau. It is only compile tested.
> > 
> 
> Thanks so much for doing this. Jerome has already absorbed these into his
> hmm-5.3 branch, along with Ralph's other fixes, so we can start testing,
> as well as reviewing, the whole set. We'll have feedback soon.

Yes, I looked at Jerome's v2's and he found a few great fixups.

My only dislike is re-introducing a READ_ONCE(mm->hmm) when a major
point of this seris was to remove that use-after-free stuff. 

But Jerome says it is a temporary defect while he works out some cross
tree API stuff.

Jason