Re: [PATCH 0/6] nouveau/hmm: add support for mapping large pages

[Matthew Wilcox]

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.