Re: [PATCH v3 3/5] dax: use common 4k zero page for dax mmap reads

[Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Wed, Jul 19, 2017 at 05:33:14PM +0200, Jan Kara wrote:
> On Wed 28-06-17 16:01:50, Ross Zwisler wrote:
> > Another major change is that we remove dax_pfn_mkwrite() from our fault
> > flow, and instead rely on the page fault itself to make the PTE dirty and
> > writeable.  The following description from the patch adding the
> > vm_insert_mixed_mkwrite() call explains this a little more:
> > 
> > ***
> >   To be able to use the common 4k zero page in DAX we need to have our PTE
> >   fault path look more like our PMD fault path where a PTE entry can be
> >   marked as dirty and writeable as it is first inserted, rather than
> >   waiting for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call.
> > 
> >   Right now we can rely on having a dax_pfn_mkwrite() call because we can
> >   distinguish between these two cases in do_wp_page():
> > 
> >   	case 1: 4k zero page => writable DAX storage
> >   	case 2: read-only DAX storage => writeable DAX storage
> > 
> >   This distinction is made by via vm_normal_page().  vm_normal_page()
> >   returns false for the common 4k zero page, though, just as it does for
> >   DAX ptes.  Instead of special casing the DAX + 4k zero page case, we will
> >   simplify our DAX PTE page fault sequence so that it matches our DAX PMD
> >   sequence, and get rid of dax_pfn_mkwrite() completely.
> > 
> >   This means that insert_pfn() needs to follow the lead of insert_pfn_pmd()
> >   and allow us to pass in a 'mkwrite' flag.  If 'mkwrite' is set
> >   insert_pfn() will do the work that was previously done by wp_page_reuse()
> >   as part of the dax_pfn_mkwrite() call path.
> > ***
> 
> Hum, thinking about this in context of this patch... So what if we have
> allocated storage, a process faults it read-only, we map it to page tables
> writeprotected. Then the process writes through mmap to the area - the code
> in handle_pte_fault() ends up in do_wp_page() if I'm reading it right.

Yep.

> Then, since we are missing ->pfn_mkwrite() handlers, the PTE will be marked
> writeable but radix tree entry stays clean - bug. Am I missing something?

I don't think we ever end up with a writeable PTE but with a clean radix tree
entry.  When we get the write fault we do a full fault through
dax_iomap_pte_fault() and dax_insert_mapping().

dax_insert_mapping() sets up the dirty radix tree entry via
dax_insert_mapping_entry() before it does anything with the page tables via
vm_insert_mixed_mkwrite().

So, this mkwrite fault path is exactly the path we would have taken if the
initial read to real storage hadn't happened, and we end up in the same end
state - with a dirty DAX radix tree entry and a writeable PTE.