[PATCH v2] mm: Fix modifying of page protection by insert_pfn_pmd()

[Matthew Wilcox]

On Thu, Apr 25, 2019 at 05:33:04PM -0700, Dan Williams wrote:
> On Thu, Apr 25, 2019 at 12:32 AM Jan Kara <jack at suse.cz> wrote:
> > > > We also call vmf_insert_pfn_pmd() in dax_insert_pfn_mkwrite() -- does
> > > > that need to change too?
> > >
> > > It wasn't clear to me that it was a problem. I think that one already
> > > happens to be pmd-aligned.
> >
> > Why would it need to be? The address is taken from vmf->address and that's
> > set up in __handle_mm_fault() like .address = address & PAGE_MASK. So I
> > don't see anything forcing PMD alignment of the virtual address...
> 
> True. So now I'm wondering if the masking should be done internal to
> the routine. Given it's prefixed vmf_ it seems to imply the api is
> prepared to take raw 'struct vm_fault' parameters. I think I'll go
> that route unless someone sees a reason to require the caller to
> handle this responsibility.

The vmf_ prefix was originally used to indicate 'returns a vm_fault_t'
instead of 'returns an errno'.  That said, I like the interpretation
you're coming up with here, and it makes me wonder if we shouldn't
change vmf_insert_pfn_pmd() to take (vmf, pfn, write) as arguments
instead of separate vma, address & pmd arguments.