Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device vma

[Logan Gunthorpe <logang@deltatee.com>]

On 2019-01-29 12:11 p.m., Jerome Glisse wrote:
> On Tue, Jan 29, 2019 at 11:36:29AM -0700, Logan Gunthorpe wrote:
>>
>>
>> On 2019-01-29 10:47 a.m., jglisse@redhat.com wrote:
>>
>>> +	/*
>>> +	 * Optional for device driver that want to allow peer to peer (p2p)
>>> +	 * mapping of their vma (which can be back by some device memory) to
>>> +	 * another device.
>>> +	 *
>>> +	 * Note that the exporting device driver might not have map anything
>>> +	 * inside the vma for the CPU but might still want to allow a peer
>>> +	 * device to access the range of memory corresponding to a range in
>>> +	 * that vma.
>>> +	 *
>>> +	 * FOR PREDICTABILITY IF DRIVER SUCCESSFULY MAP A RANGE ONCE FOR A
>>> +	 * DEVICE THEN FURTHER MAPPING OF THE SAME IF THE VMA IS STILL VALID
>>> +	 * SHOULD ALSO BE SUCCESSFUL. Following this rule allow the importing
>>> +	 * device to map once during setup and report any failure at that time
>>> +	 * to the userspace. Further mapping of the same range might happen
>>> +	 * after mmu notifier invalidation over the range. The exporting device
>>> +	 * can use this to move things around (defrag BAR space for instance)
>>> +	 * or do other similar task.
>>> +	 *
>>> +	 * IMPORTER MUST OBEY mmu_notifier NOTIFICATION AND CALL p2p_unmap()
>>> +	 * WHEN A NOTIFIER IS CALL FOR THE RANGE ! THIS CAN HAPPEN AT ANY
>>> +	 * POINT IN TIME WITH NO LOCK HELD.
>>> +	 *
>>> +	 * In below function, the device argument is the importing device,
>>> +	 * the exporting device is the device to which the vma belongs.
>>> +	 */
>>> +	long (*p2p_map)(struct vm_area_struct *vma,
>>> +			struct device *device,
>>> +			unsigned long start,
>>> +			unsigned long end,
>>> +			dma_addr_t *pa,
>>> +			bool write);
>>> +	long (*p2p_unmap)(struct vm_area_struct *vma,
>>> +			  struct device *device,
>>> +			  unsigned long start,
>>> +			  unsigned long end,
>>> +			  dma_addr_t *pa);
>>
>> I don't understand why we need new p2p_[un]map function pointers for
>> this. In subsequent patches, they never appear to be set anywhere and
>> are only called by the HMM code. I'd have expected it to be called by
>> some core VMA code and set by HMM as that's what vm_operations_struct is
>> for.
>>
>> But the code as all very confusing, hard to follow and seems to be
>> missing significant chunks. So I'm not really sure what is going on.
> 
> It is set by device driver when userspace do mmap(fd) where fd comes
> from open("/dev/somedevicefile"). So it is set by device driver. HMM
> has nothing to do with this. It must be set by device driver mmap
> call back (mmap callback of struct file_operations). For this patch
> you can completely ignore all the HMM patches. Maybe posting this as
> 2 separate patchset would make it clearer.
> 
> For instance see [1] for how a non HMM driver can export its memory
> by just setting those callback. Note that a proper implementation of
> this should also include some kind of driver policy on what to allow
> to map and what to not allow ... All this is driver specific in any
> way.

I'd suggest [1] should be a part of the patchset so we can actually see
a user of the stuff you're adding.

But it still doesn't explain everything as without the HMM code nothing
calls the new vm_ops. And there's still no callers for the p2p_test
functions you added. And I still don't understand why we need the new
vm_ops or who calls them and when. Why can't drivers use the existing
'fault' vm_op and call a new helper function to map p2p when appropriate
or a different helper function to map a large range in its mmap
operation? Just like regular mmap code...

Logan