Date:   Wed, 22 May 2019 13:48:52 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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>
Subject: Re: [PATCH v4 0/1] Use HMM for ODP v4

On Tue, May 21, 2019 at 09:52:25PM -0300, Jason Gunthorpe wrote:
> On Tue, May 21, 2019 at 04:53:22PM -0400, Jerome Glisse wrote:
> > On Mon, May 06, 2019 at 04:56:57PM -0300, Jason Gunthorpe wrote:
> > > On Thu, Apr 11, 2019 at 02:13:13PM -0400, jglisse@...hat.com wrote:
> > > > From: Jérôme Glisse <jglisse@...hat.com>
> > > > 
> > > > Just fixed Kconfig and build when ODP was not enabled, other than that
> > > > this is the same as v3. Here is previous cover letter:
> > > > 
> > > > Git tree with all prerequisite:
> > > > https://cgit.freedesktop.org/~glisse/linux/log/?h=rdma-odp-hmm-v4
> > > > 
> > > > This patchset convert RDMA ODP to use HMM underneath this is motivated
> > > > by stronger code sharing for same feature (share virtual memory SVM or
> > > > Share Virtual Address SVA) and also stronger integration with mm code to
> > > > achieve that. It depends on HMM patchset posted for inclusion in 5.2 [2]
> > > > and [3].
> > > > 
> > > > It has been tested with pingpong test with -o and others flags to test
> > > > different size/features associated with ODP.
> > > > 
> > > > Moreover they are some features of HMM in the works like peer to peer
> > > > support, fast CPU page table snapshot, fast IOMMU mapping update ...
> > > > It will be easier for RDMA devices with ODP to leverage those if they
> > > > use HMM underneath.
> > > > 
> > > > Quick summary of what HMM is:
> > > >     HMM is a toolbox for device driver to implement software support for
> > > >     Share Virtual Memory (SVM). Not only it provides helpers to mirror a
> > > >     process address space on a device (hmm_mirror). It also provides
> > > >     helper to allow to use device memory to back regular valid virtual
> > > >     address of a process (any valid mmap that is not an mmap of a device
> > > >     or a DAX mapping). They are two kinds of device memory. Private memory
> > > >     that is not accessible to CPU because it does not have all the expected
> > > >     properties (this is for all PCIE devices) or public memory which can
> > > >     also be access by CPU without restriction (with OpenCAPI or CCIX or
> > > >     similar cache-coherent and atomic inter-connect).
> > > > 
> > > >     Device driver can use each of HMM tools separatly. You do not have to
> > > >     use all the tools it provides.
> > > > 
> > > > For RDMA device i do not expect a need to use the device memory support
> > > > of HMM. This device memory support is geared toward accelerator like GPU.
> > > > 
> > > > 
> > > > You can find a branch [1] with all the prerequisite in. This patch is on
> > > > top of rdma-next with the HMM patchset [2] and mmu notifier patchset [3]
> > > > applied on top of it.
> > > > 
> > > > [1] https://cgit.freedesktop.org/~glisse/linux/log/?h=rdma-odp-hmm-v4
> > > > [2] https://lkml.org/lkml/2019/4/3/1032
> > > > [3] https://lkml.org/lkml/2019/3/26/900
> > > 
> > > Jerome, please let me know if these dependent series are merged during
> > > the first week of the merge window.
> > > 
> > > This patch has been tested and could go along next week if the
> > > dependencies are met.
> > > 
> > 
> > So attached is a rebase on top of 5.2-rc1, i have tested with pingpong
> > (prefetch and not and different sizes). Seems to work ok.
> 
> Urk, it already doesn't apply to the rdma tree :(
> 
> The conflicts are a little more extensive than I'd prefer to handle..
> Can I ask you to rebase it on top of this branch please:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/rdma/rdma.git/log/?h=wip/jgg-for-next
> 
> Specifically it conflicts with this patch:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/rdma/rdma.git/commit/?h=wip/jgg-for-next&id=d2183c6f1958e6b6dfdde279f4cee04280710e34
> 
> > +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.

> 
> > +	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.

Now if we race with a destruction (ie all process that referenced the
mm struct are dead/dying) hmm struct will be marked as dead this is
tested by hmm_mirror_mm_is_alive() and also by hmm_range_snapshot()
(which is call by hmm_range_dma_map()). In other word if mm->mm_users
is zero then hmm will be mark as dead.

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".



> 
> > @@ -603,11 +603,29 @@ static int pagefault_mr(struct mlx5_ib_dev *dev, struct mlx5_ib_mr *mr,
> >  
> >  next_mr:
> >  	size = min_t(size_t, bcnt, ib_umem_end(&odp->umem) - io_virt);
> > -
> >  	page_shift = mr->umem->page_shift;
> >  	page_mask = ~(BIT(page_shift) - 1);
> > +	/*
> > +	 * We need to align io_virt on page size so off is the extra bytes we
> > +	 * will be faulting and fault_size is the page aligned size we are
> > +	 * faulting.
> > +	 */
> > +	io_virt = io_virt & page_mask;
> > +	off = (io_virt & (~page_mask));
> > +	fault_size = ALIGN(size + off, 1UL << page_shift);
> > +
> > +	if (io_virt < ib_umem_start(&odp->umem))
> > +		return -EINVAL;
> > +
> >  	start_idx = (io_virt - (mr->mmkey.iova & page_mask)) >> page_shift;
> > -	access_mask = ODP_READ_ALLOWED_BIT;
> > +
> > +	if (odp_mr->per_mm == NULL || odp_mr->per_mm->mm == NULL)
> > +		return -ENOENT;
> 
> How can this happen? Where is the locking?
> 
> per_mm is supposed to outlive any odp_mr's the refer to it, and the mm
> is supposed to remain grab'd as long as the per_mm exists..

This can not happen removed the test.

> 
> > diff --git a/include/rdma/ib_umem_odp.h b/include/rdma/ib_umem_odp.h
> > index eeec4e53c448..70b2df8e5a6c 100644
> > +++ b/include/rdma/ib_umem_odp.h
> > @@ -36,6 +36,7 @@
> >  #include <rdma/ib_umem.h>
> >  #include <rdma/ib_verbs.h>
> >  #include <linux/interval_tree.h>
> > +#include <linux/hmm.h>
> >  
> >  struct umem_odp_node {
> >  	u64 __subtree_last;
> > @@ -47,11 +48,11 @@ struct ib_umem_odp {
> >  	struct ib_ucontext_per_mm *per_mm;
> >  
> >  	/*
> > -	 * 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.

Cheers,
Jérôme

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