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.
