Re: [PATCH v2 2/5] mm/hmm: add output flags for PMD/PUD page mapping

[Jason Gunthorpe]

On Tue, Jun 30, 2020 at 10:23:43PM +0100, Matthew Wilcox wrote:
> On Tue, Jun 30, 2020 at 12:57:34PM -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 a
> > compound page is mapped by the CPU using a larger page size. Without
> > this information, the caller can't safely use a large device PTE to map
> > the compound page because the CPU might be using smaller PTEs with
> > different read/write permissions.
> > 
> > Add two new output flags to indicate the mapping size (PMD or PUD sized)
> > so that callers know the pages are being mapped with consistent permissions
> > and a large device page table mapping can be used if one is available.
> 
> The problem I have with this is that PTE/PMD/PUD are not the only choices
> for how the CPU might choose to map something.  For example, ARM has
> the ability to map 64kB pages using 16 consecutive page table entries
> (marked specially so the CPU knows to use a single TLB entry for the
> 64kB range).  Some other CPUs have similar capabilities.

Sure, but at the moment this is the only thing hmm_range_fault() is able
to detect and set..

> I'd rather you encoded the order of the mapping in the flags (eg a
> number between 0 and 31) so that we have the flexibility in the future
> to describe how memory is mapped.

How about some hmm_get_mapping_order() API, we can keep the flags that
match the implementation but the driver facing API will see something
more general?

Jason