Date:   Wed, 22 May 2019 16:22:19 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
        Leon Romanovsky <leonro@...lanox.com>,
        Doug Ledford <dledford@...hat.com>,
        Artemy Kovalyov <artemyko@...lanox.com>,
        Moni Shoua <monis@...lanox.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Kaike Wan <kaike.wan@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        linux-mm@...ck.org
Subject: Re: [PATCH v4 0/1] Use HMM for ODP v4

On Wed, May 22, 2019 at 01:48:52PM -0400, Jerome Glisse wrote:

> > > +long ib_umem_odp_map_dma_pages(struct ib_umem_odp *umem_odp,
> > > +			       struct hmm_range *range)
> > >  {
> > > +	struct device *device = umem_odp->umem.context->device->dma_device;
> > > +	struct ib_ucontext_per_mm *per_mm = umem_odp->per_mm;
> > >  	struct ib_umem *umem = &umem_odp->umem;
> > > -	struct task_struct *owning_process  = NULL;
> > > -	struct mm_struct *owning_mm = umem_odp->umem.owning_mm;
> > > -	struct page       **local_page_list = NULL;
> > > -	u64 page_mask, off;
> > > -	int j, k, ret = 0, start_idx, npages = 0, page_shift;
> > > -	unsigned int flags = 0;
> > > -	phys_addr_t p = 0;
> > > -
> > > -	if (access_mask == 0)
> > > +	struct mm_struct *mm = per_mm->mm;
> > > +	unsigned long idx, npages;
> > > +	long ret;
> > > +
> > > +	if (mm == NULL)
> > > +		return -ENOENT;
> > > +
> > > +	/* Only drivers with invalidate support can use this function. */
> > > +	if (!umem->context->invalidate_range)
> > >  		return -EINVAL;
> > >  
> > > -	if (user_virt < ib_umem_start(umem) ||
> > > -	    user_virt + bcnt > ib_umem_end(umem))
> > > -		return -EFAULT;
> > > +	/* Sanity checks. */
> > > +	if (range->default_flags == 0)
> > > +		return -EINVAL;
> > >  
> > > -	local_page_list = (struct page **)__get_free_page(GFP_KERNEL);
> > > -	if (!local_page_list)
> > > -		return -ENOMEM;
> > > +	if (range->start < ib_umem_start(umem) ||
> > > +	    range->end > ib_umem_end(umem))
> > > +		return -EINVAL;
> > >  
> > > -	page_shift = umem->page_shift;
> > > -	page_mask = ~(BIT(page_shift) - 1);
> > > -	off = user_virt & (~page_mask);
> > > -	user_virt = user_virt & page_mask;
> > > -	bcnt += off; /* Charge for the first page offset as well. */
> > > +	idx = (range->start - ib_umem_start(umem)) >> umem->page_shift;
> > 
> > Is this math OK? What is supposed to happen if the range->start is not
> > page aligned to the internal page size?
> 
> range->start is align on 1 << page_shift boundary within pagefault_mr
> thus the above math is ok. We can add a BUG_ON() and comments if you
> want.

OK

> > > +	range->pfns = &umem_odp->pfns[idx];
> > > +	range->pfn_shift = ODP_FLAGS_BITS;
> > > +	range->values = odp_hmm_values;
> > > +	range->flags = odp_hmm_flags;
> > >  
> > >  	/*
> > > -	 * owning_process is allowed to be NULL, this means somehow the mm is
> > > -	 * existing beyond the lifetime of the originating process.. Presumably
> > > -	 * mmget_not_zero will fail in this case.
> > > +	 * If mm is dying just bail out early without trying to take mmap_sem.
> > > +	 * Note that this might race with mm destruction but that is fine the
> > > +	 * is properly refcounted so are all HMM structure.
> > >  	 */
> > > -	owning_process = get_pid_task(umem_odp->per_mm->tgid, PIDTYPE_PID);
> > > -	if (!owning_process || !mmget_not_zero(owning_mm)) {
> > 
> > But we are not in a HMM context here, and per_mm is not a HMM
> > structure. 
> > 
> > So why is mm suddenly guarenteed valid? It was a bug report that
> > triggered the race the mmget_not_zero is fixing, so I need a better
> > explanation why it is now safe. From what I see the hmm_range_fault
> > is doing stuff like find_vma without an active mmget??
> 
> So the mm struct can not go away as long as we hold a reference on
> the hmm struct and we hold a reference on it through both hmm_mirror
> and hmm_range struct. So struct mm can not go away and thus it is
> safe to try to take its mmap_sem.

This was always true here, though, so long as the umem_odp exists the
the mm has a grab on it. But a grab is not a get..

The point here was the old code needed an mmget() in order to do
get_user_pages_remote()

If hmm does not need an external mmget() then fine, we delete this
stuff and rely on hmm.

But I don't think that is true as we have:

          CPU 0                                           CPU1
                                                       mmput()
                       				        __mmput()
							 exit_mmap()
down_read(&mm->mmap_sem);
hmm_range_dma_map(range, device,..
  ret = hmm_range_fault(range, block);
     if (hmm->mm == NULL || hmm->dead)
							   mmu_notifier_release()
							     hmm->dead = true
     vma = find_vma(hmm->mm, start);
        .. rb traversal ..                                 while (vma) remove_vma()

*goes boom*

I think this is violating the basic constraint of the mm by acting on
a mm's VMA's without holding a mmget() to prevent concurrent
destruction.

In other words, mmput() destruction does not respect the mmap_sem - so
holding the mmap sem alone is not enough locking.

The unlucked hmm->dead simply can't save this. Frankly every time I
look a struct with 'dead' in it, I find races like this.

Thus we should put the mmget_notzero back in.

I saw some other funky looking stuff in hmm as well..

> Hence it is safe to take mmap_sem and it is safe to call in hmm, if
> mm have been kill it will return EFAULT and this will propagate to
> RDMA.
 
> As per_mm i removed the per_mm->mm = NULL from release so that it is
> always safe to use that field even in face of racing mm "killing".

Yes, that certainly wasn't good.

> > > -	 * An array of the pages included in the on-demand paging umem.
> > > -	 * Indices of pages that are currently not mapped into the device will
> > > -	 * contain NULL.
> > > +	 * An array of the pages included in the on-demand paging umem. Indices
> > > +	 * of pages that are currently not mapped into the device will contain
> > > +	 * 0.
> > >  	 */
> > > -	struct page		**page_list;
> > > +	uint64_t *pfns;
> > 
> > Are these actually pfns, or are they mangled with some shift? (what is range->pfn_shift?)
> 
> They are not pfns they have flags (hence range->pfn_shift) at the
> bottoms i just do not have a better name for this.

I think you need to have a better name then

Jason

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