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!
