Re: [PATCH v2 1/3] mm: add vm_insert_mixed_mkwrite()

From: Ross Zwisler
Date: Fri Jun 16 2017 - 15:44:46 EST

On Thu, Jun 15, 2017 at 04:42:04PM +0200, Jan Kara wrote:
> On Wed 14-06-17 11:22:09, 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>
> 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!