Re: [HMM v13 06/18] mm/ZONE_DEVICE/unaddressable: add special swap for unaddressable

[Balbir Singh]

On 21/11/16 16:05, Jerome Glisse wrote:
> On Mon, Nov 21, 2016 at 01:06:45PM +1100, Balbir Singh wrote:
>>
>>
>> On 19/11/16 05:18, Jérôme Glisse wrote:
>>> To allow use of device un-addressable memory inside a process add a
>>> special swap type. Also add a new callback to handle page fault on
>>> such entry.
>>>
>>> Signed-off-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
>>> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>>> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>>> ---
>>>  fs/proc/task_mmu.c       | 10 +++++++-
>>>  include/linux/memremap.h |  5 ++++
>>>  include/linux/swap.h     | 18 ++++++++++---
>>>  include/linux/swapops.h  | 67 ++++++++++++++++++++++++++++++++++++++++++++++++
>>>  kernel/memremap.c        | 14 ++++++++++
>>>  mm/Kconfig               | 12 +++++++++
>>>  mm/memory.c              | 24 +++++++++++++++++
>>>  mm/mprotect.c            | 12 +++++++++
>>>  8 files changed, 158 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>>> index 6909582..0726d39 100644
>>> --- a/fs/proc/task_mmu.c
>>> +++ b/fs/proc/task_mmu.c
>>> @@ -544,8 +544,11 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
>>>  			} else {
>>>  				mss->swap_pss += (u64)PAGE_SIZE << PSS_SHIFT;
>>>  			}
>>> -		} else if (is_migration_entry(swpent))
>>> +		} else if (is_migration_entry(swpent)) {
>>>  			page = migration_entry_to_page(swpent);
>>> +		} else if (is_device_entry(swpent)) {
>>> +			page = device_entry_to_page(swpent);
>>> +		}
>>
>>
>> So the reason there is a device swap entry for a page belonging to a user process is
>> that it is in the middle of migration or is it always that a swap entry represents
>> unaddressable memory belonging to a GPU device, but its tracked in the page table
>> entries of the process.
> 
> For page being migrated i use the existing special migration pte entry. This new device
> special swap entry is only for unaddressable memory belonging to a device (GPU or any
> else). We need to keep track of those inside the CPU page table. Using a new special
> swap entry is the easiest way with the minimum amount of change to core mm.
> 

Thanks, makes sense

> [...]
> 
>>> +#ifdef CONFIG_DEVICE_UNADDRESSABLE
>>> +static inline swp_entry_t make_device_entry(struct page *page, bool write)
>>> +{
>>> +	return swp_entry(write?SWP_DEVICE_WRITE:SWP_DEVICE, page_to_pfn(page));
>>
>> Code style checks
> 
> I was trying to balance against 79 columns break rule :)
> 
> [...]
> 
>>> +		} else if (is_device_entry(entry)) {
>>> +			page = device_entry_to_page(entry);
>>> +
>>> +			get_page(page);
>>> +			rss[mm_counter(page)]++;
>>
>> Why does rss count go up?
> 
> I wanted the device page to be treated like any other page. There is an argument
> to be made against and for doing that. Do you have strong argument for not doing
> this ?
> 

Yes, It will end up confusing rss accounting IMHO. If a task is using a lot of
pages on the GPU, should be it a candidate for OOM based on it's RSS for example?

> [...]
> 
>>> @@ -2536,6 +2557,9 @@ int do_swap_page(struct fault_env *fe, pte_t orig_pte)
>>>  	if (unlikely(non_swap_entry(entry))) {
>>>  		if (is_migration_entry(entry)) {
>>>  			migration_entry_wait(vma->vm_mm, fe->pmd, fe->address);
>>> +		} else if (is_device_entry(entry)) {
>>> +			ret = device_entry_fault(vma, fe->address, entry,
>>> +						 fe->flags, fe->pmd);
>>
>> What does device_entry_fault() actually do here?
> 
> Well it is a special fault handler, it must migrate the memory back to some place
> where the CPU can access it. It only matter for unaddressable memory.

So effectively swap the page back in, chances are it can ping pong ...but I was wondering if we can
tell the GPU that the CPU is accessing these pages as well. I presume any operation that causes
memory access - core dump will swap back in things from the HMM side onto the CPU side.

> 
>>>  		} else if (is_hwpoison_entry(entry)) {
>>>  			ret = VM_FAULT_HWPOISON;
>>>  		} else {
>>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>>> index 1bc1eb3..70aff3a 100644
>>> --- a/mm/mprotect.c
>>> +++ b/mm/mprotect.c
>>> @@ -139,6 +139,18 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>>>  
>>>  				pages++;
>>>  			}
>>> +
>>> +			if (is_write_device_entry(entry)) {
>>> +				pte_t newpte;
>>> +
>>> +				make_device_entry_read(&entry);
>>> +				newpte = swp_entry_to_pte(entry);
>>> +				if (pte_swp_soft_dirty(oldpte))
>>> +					newpte = pte_swp_mksoft_dirty(newpte);
>>> +				set_pte_at(mm, addr, pte, newpte);
>>> +
>>> +				pages++;
>>> +			}
>>
>> Does it make sense to call mprotect() on device memory ranges?
> 
> There is nothing special about vma that containt device memory. They can be
> private anonymous, share, file back ... So any existing memory syscall must
> behave as expected. This is really just like any other page except that CPU
> can not access it.

I understand that, but what would marking it as R/O when the GPU is in the middle
of write mean? I would also worry about passing "executable" pages over to the
other side.

Balbir Singh.