Nouveau - May 2020 - [PATCH 4/6] mm/hmm: add output flag for compound page mapping

On Fri, May 08, 2020 at 12:20:07PM -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.
> Add 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.
Why do you need the flag?  The caller should be able to just use
page_size() (or willys new thp_size helper).

On 5/8/20 12:51 PM, Christoph Hellwig wrote:
> On Fri, May 08, 2020 at 12:20:07PM -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.
>> Add 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.
> 
> Why do you need the flag?  The caller should be able to just use
> page_size() (or willys new thp_size helper).
> 
The question is whether or not a large page can be mapped with smaller
page table entries with different permissions. If one process has a 2MB
page mapped with 4K PTEs with different read/write permissions, I don't
think
it would be OK for a device to map the whole 2MB with write access enabled.
The flag is supposed to indicate that the whole page can be mapped by the
device with the indicated read/write permissions.

On 8 May 2020, at 16:06, Ralph Campbell wrote:
> On 5/8/20 12:51 PM, Christoph Hellwig wrote:
>> On Fri, May 08, 2020 at 12:20:07PM -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.
>>> Add 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.
>>
>> Why do you need the flag?  The caller should be able to just use
>> page_size() (or willys new thp_size helper).
>>
>
> The question is whether or not a large page can be mapped with smaller
> page table entries with different permissions. If one process has a 2MB
> page mapped with 4K PTEs with different read/write permissions, I don't
think
> it would be OK for a device to map the whole 2MB with write access enabled.
> The flag is supposed to indicate that the whole page can be mapped by the
> device with the indicated read/write permissions.
If hmm_range_fault() only walks one VMA at a time, you would not have this
permission
issue, right? Since all pages from one VMA should have the same permission.
But it seems that hmm_range_fault() deals with pages across multiple VMAs.
Maybe we should make hmm_range_fault() bail out early when it encounters
a VMA with a different permission than the existing ones?


?
Best Regards,
Yan Zi
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