On Mon, May 22, 2017 at 04:37:48PM +0200, Jan Kara wrote: > On Thu 18-05-17 15:29:39, Ross Zwisler wrote: > > On Thu, May 18, 2017 at 09:50:37AM +0200, Jan Kara wrote: > > > On Wed 17-05-17 11:16:39, Ross Zwisler wrote: <> > > > The first scenario seems to be possible. dax_iomap_pmd_fault() will create > > > PMD entry in the radix tree. Then dax_iomap_pte_fault() will come, do > > > grab_mapping_entry(), there it sees entry is PMD but we are doing PTE fault > > > so I'd think that pmd_downgrade = true... But actually the condition there > > > doesn't trigger in this case. And that's a catch that although we asked > > > grab_mapping_entry() for PTE, we've got PMD back and that screws us later. > > > > Yep, it was a concious decision when implementing the PMD support to allow one > > thread to use PMDs and another to use PTEs in the same range, as long as the > > thread faulting in PMDs is the first to insert into the radix tree. A PMD > > radix tree entry will be inserted and used for locking and dirty tracking, and > > each thread or process can fault in either PTEs or PMDs into its own address > > space as needed. > > Well, for *threads* it doesn't really make good sense to mix PMDs and PTEs > as they share page tables. However for *processes* it makes some sense to > allow one process to use PTEs and another process to use PMDs. And I > remember we were discussing this in the past. Ugh, I was super sloppy with my use of "thread" and "process" in my previous email. Sorry, and thanks for the clarifications. I think we're on the same page, even if I had trouble articulating it. :) > So normal fault path uses alloc_set_pte() for installing new PTE. And that > uses pte_alloc_one_map() which checks whether PMD is still suitable for > inserting a PTE. If not, we return VM_FAULT_NOPAGE. Probably it would be > cleanest to factor our common parts of PTE and PMD insertion so that we can > use these functions both from DAX and generic fault paths. Makes sense, thanks.