Nouveau - May 2020 - [PATCH 0/6] nouveau/hmm: add support for mapping large pages

On 5/8/20 12:59 PM, Matthew Wilcox wrote:
> On Fri, May 08, 2020 at 12:20:03PM -0700, Ralph Campbell wrote:
>> hmm_range_fault() returns an array of page frame numbers and flags for
>> how the pages are mapped in the requested process' page tables. The
PFN
>> can be used to get the struct page with hmm_pfn_to_page() and the page
size
>> order can be determined with compound_order(page) but if the page is
larger
>> than order 0 (PAGE_SIZE), there is no indication that the page is
mapped
>> using a larger page size. To be fully general, hmm_range_fault() would
need
>> to return the mapping size to handle cases like a 1GB compound page
being
>> mapped with 2MB PMD entries. However, the most common case is the
mapping
>> size the same as the underlying compound page size.
>> This series adds a new output flag to indicate this so that callers
know it
>> is safe to use a large device page table mapping if one is available.
>> Nouveau and the HMM tests are updated to use the new flag.
> 
> This explanation doesn't make any sense.  It doesn't matter how
somebody
> else has it mapped; if it's a PMD-sized page, you can map it with a
> 2MB mapping.
> 
Sure, the I/O will work OK, but is it safe?
Copy on write isn't an issue? splitting a PMD in one process due to
mprotect of a shared page will cause other process' page tables to be split
the same way?
Recall that these are system memory pages that could be THPs, shmem, hugetlbfs,
mmap shared file pages, etc.

On Fri, May 08, 2020 at 01:17:55PM -0700, Ralph Campbell
wrote:
> On 5/8/20 12:59 PM, Matthew Wilcox wrote:
> > On Fri, May 08, 2020 at 12:20:03PM -0700, Ralph Campbell wrote:
> > > hmm_range_fault() returns an array of page frame numbers and
flags for
> > > how the pages are mapped in the requested process' page
tables. The PFN
> > > can be used to get the struct page with hmm_pfn_to_page() and the
page size
> > > order can be determined with compound_order(page) but if the page
is larger
> > > than order 0 (PAGE_SIZE), there is no indication that the page is
mapped
> > > using a larger page size. To be fully general, hmm_range_fault()
would need
> > > to return the mapping size to handle cases like a 1GB compound
page being
> > > mapped with 2MB PMD entries. However, the most common case is the
mapping
> > > size the same as the underlying compound page size.
> > > This series adds a new output flag to indicate this so that
callers know it
> > > is safe to use a large device page table mapping if one is
available.
> > > Nouveau and the HMM tests are updated to use the new flag.
> > 
> > This explanation doesn't make any sense.  It doesn't matter
how somebody
> > else has it mapped; if it's a PMD-sized page, you can map it with
a
> > 2MB mapping.
> 
> Sure, the I/O will work OK, but is it safe?
> Copy on write isn't an issue? splitting a PMD in one process due to
> mprotect of a shared page will cause other process' page tables to be
split
> the same way?
Are you saying that if you call this function on an address range of a
process which has done COW of a single page in the middle of a THP,
you want to return with this flag clear, but if the THP is still intact,
you want to set this flag?
> Recall that these are system memory pages that could be THPs, shmem,
hugetlbfs,
> mmap shared file pages, etc.

On 5/8/20 8:17 PM, Matthew Wilcox wrote:
> On Fri, May 08, 2020 at 01:17:55PM -0700, Ralph Campbell wrote:
>> On 5/8/20 12:59 PM, Matthew Wilcox wrote:
>>> On Fri, May 08, 2020 at 12:20:03PM -0700, Ralph Campbell wrote:
>>>> hmm_range_fault() returns an array of page frame numbers and
flags for
>>>> how the pages are mapped in the requested process' page
tables. The PFN
>>>> can be used to get the struct page with hmm_pfn_to_page() and
the page size
>>>> order can be determined with compound_order(page) but if the
page is larger
>>>> than order 0 (PAGE_SIZE), there is no indication that the page
is mapped
>>>> using a larger page size. To be fully general,
hmm_range_fault() would need
>>>> to return the mapping size to handle cases like a 1GB compound
page being
>>>> mapped with 2MB PMD entries. However, the most common case is
the mapping
>>>> size the same as the underlying compound page size.
>>>> This series adds a new output flag to indicate this so that
callers know it
>>>> is safe to use a large device page table mapping if one is
available.
>>>> Nouveau and the HMM tests are updated to use the new flag.
>>>
>>> This explanation doesn't make any sense.  It doesn't matter
how somebody
>>> else has it mapped; if it's a PMD-sized page, you can map it
with a
>>> 2MB mapping.
>>
>> Sure, the I/O will work OK, but is it safe?
>> Copy on write isn't an issue? splitting a PMD in one process due to
>> mprotect of a shared page will cause other process' page tables to
be split
>> the same way?
> 
> Are you saying that if you call this function on an address range of a
> process which has done COW of a single page in the middle of a THP,
> you want to return with this flag clear, but if the THP is still intact,
> you want to set this flag?
Correct. I want the GPU to see the same faults that the CPU would see when
trying
to access the same addresses. All faults, whether from CPU or GPU, end up
calling
handle_mm_fault() to handle the fault and update the GPU/CPU page tables.
>> Recall that these are system memory pages that could be THPs, shmem,
hugetlbfs,
>> mmap shared file pages, etc.