Re: [PATCH v3 1/5] mm: add vm_insert_mixed_mkwrite()

[Ross Zwisler]

On Wed, Jul 19, 2017 at 04:16:59PM +0200, Jan Kara wrote:
> On Wed 28-06-17 16:01:48, Ross Zwisler wrote:
> > 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.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> 
> Just one small comment below.
> 
> > @@ -1658,14 +1658,26 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
> >  	if (!pte)
> >  		goto out;
> >  	retval = -EBUSY;
> > -	if (!pte_none(*pte))
> > -		goto out_unlock;
> > +	if (!pte_none(*pte)) {
> > +		if (mkwrite) {
> > +			entry = *pte;
> > +			goto out_mkwrite;
> 
> Can we maybe check here that (pte_pfn(*pte) == pfn_t_to_pfn(pfn)) and
> return -EBUSY otherwise? That way we are sure insert_pfn() isn't doing
> anything we don't expect 

Sure, that's fine.  I'll add it as a WARN_ON_ONCE() so it's a very loud
failure.  If the pfns don't match I think we're insane (and would have been
insane prior to this patch series as well) because we are getting a page fault
and somehow have a different PFN already mapped at that location.

> and if I understand the code right, we need to
> invalidate all zero page mappings at given file offset (via
> unmap_mapping_range()) before mapping an allocated block there and thus the
> case of filling the hole won't be affected by this?

Correct.  Here's the call tree if we already have a zero page mapped and are
now faulting in an allocated block:

dax_iomap_pte_fault()
  dax_insert_mapping()
    dax_insert_mapping_entry()
      unmap_mapping_range() for our zero page
    vm_insert_mixed_mkwrite() installs the new PTE. We have pte_none(), so we
    				skip the new mkwrite goto in insert_pfn().