On Fri 21-07-17 11:44:05, Ross Zwisler wrote: > On Wed, Jul 19, 2017 at 03:58:31PM -0600, Ross Zwisler wrote: > > On Wed, Jul 19, 2017 at 11:51:12AM -0600, Ross Zwisler wrote: > > > On Wed, Jul 19, 2017 at 04:16:59PM +0200, Jan Kara wrote: > > > > On Wed 28-06-17 16:01:48, 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> > > > > > > > > Just one small comment below. > > > > > > > > > @@ -1658,14 +1658,26 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr, > > > > > if (!pte) > > > > > goto out; > > > > > retval = -EBUSY; > > > > > - if (!pte_none(*pte)) > > > > > - goto out_unlock; > > > > > + if (!pte_none(*pte)) { > > > > > + if (mkwrite) { > > > > > + entry = *pte; > > > > > + goto out_mkwrite; > > > > > > > > Can we maybe check here that (pte_pfn(*pte) == pfn_t_to_pfn(pfn)) and > > > > return -EBUSY otherwise? That way we are sure insert_pfn() isn't doing > > > > anything we don't expect > > > > > > Sure, that's fine. I'll add it as a WARN_ON_ONCE() so it's a very loud > > > failure. If the pfns don't match I think we're insane (and would have been > > > insane prior to this patch series as well) because we are getting a page fault > > > and somehow have a different PFN already mapped at that location. > > > > Umm...well, I added the warning, and during my regression testing hit a case > > where the PFNs didn't match. (generic/437 with both ext4 & XFS) > > > > I've verified that this behavior happens with vanilla v4.12, so it's not a new > > condition introduced by my patch. > > > > I'm off tracking that down - there's a bug lurking somewhere, I think. > > Actually, I think we're fine. What was happening was that two faults were > racing for a private mapping. One was installing a RW PTE for the COW page > cache page via wp_page_copy(), and the second was trying to install a > read-only PTE in insert_pfn(). The PFNs don't match because the two faults > are trying to map very different PTEs - one for DAX storage, one for a page > cache page. OK, so two threads (sharing page tables) were doing read and write fault at the same offset of a private mapping. OK, makes sense. > This collision is handled by insert_pfn() by just returning -EBUSY, which will > bail out of the fault and either re-fault if necessary, or use the PTE that > the other thread installed. For the case I described above I think both > faults will just happily use the page cache page, and the RO DAX fault won't > be retried. > > I think this is fine, and I'll preserve this behavior as you suggest in the > mkwrite case by validating that the PTE is what we think it should be after we > grab the PTL. Yeah, that seems to essential for the races of faults in private mappings to work as they should. Thanks for analysing this! Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>