On Thu, Jun 15, 2017 at 04:42:04PM +0200, Jan Kara wrote:

On Wed 14-06-17 11:22:09, Ross Zwisler wrote:

quoted

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 <redacted>

So I agree that getting rid of dax_pfn_mkwrite() and using fault handler in
that case is a way to go. However I somewhat dislike the
vm_insert_mixed_mkwrite() thing - it looks like a hack - and I'm aware that
we have a similar thing for PMD which is ugly as well. Besides being ugly
I'm also concerned that when 'mkwrite' is set, we just silently overwrite
whatever PTE was installed at that position. Not that I'd see how that
could screw us for DAX but still a concern that e.g. some PTE flag could
get discarded by this is there... In fact, for !HAVE_PTE_SPECIAL
architectures, you will leak zero page references by just overwriting the
PTE - for those archs you really need to unmap zero page before replacing
PTE (and the same for PMD I suppose).

So how about some vmf_insert_pfn(vmf, pe_size, pfn) helper that would
properly detect PTE / PMD case, read / write case etc., check that PTE did
not change from orig_pte, and handle all the nasty details instead of
messing with insert_pfn?

								Honza

Sounds good, I'll figure this out for v3.

Thanks for the review!

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