On Fri, 2015-05-29 at 11:19 -0700, Dan Williams wrote: > On Fri, May 29, 2015 at 8:03 AM, Toshi Kani <toshi.kani@xxxxxx> wrote: > > On Fri, 2015-05-29 at 07:43 -0700, Dan Williams wrote: > >> On Fri, May 29, 2015 at 2:11 AM, Borislav Petkov <bp@xxxxxxxxx> wrote: > >> > On Wed, May 27, 2015 at 09:19:04AM -0600, Toshi Kani wrote: > >> >> The pmem driver maps NVDIMM with ioremap_nocache() as we cannot : > >> >> - pmem->virt_addr = ioremap_nocache(pmem->phys_addr, pmem->size); > >> >> + pmem->virt_addr = ioremap_wt(pmem->phys_addr, pmem->size); > >> >> if (!pmem->virt_addr) > >> >> goto out_release_region; > >> > > >> > Dan, Ross, what about this one? > >> > > >> > ACK to pick it up as a temporary solution? > >> > >> I see that is_new_memtype_allowed() is updated to disallow some > >> combinations, but the manual seems to imply any mixing of memory types > >> is unsupported. Which worries me even in the current code where we > >> have uncached mappings in the driver, and potentially cached DAX > >> mappings handed out to userspace. > > > > is_new_memtype_allowed() is not to allow some combinations of mixing of > > memory types. When it is allowed, the requested type of ioremap_xxx() > > is changed to match with the existing map type, so that mixing of memory > > types does not happen. > > Yes, but now if the caller was expecting one memory type and gets > another one that is something I think the driver would want to know. > At a minimum I don't think we want to get emails about pmem driver > performance problems when someone's platform is silently degrading WB > to UC for example. The pmem driver creates an ioremap map to an NVDIMM range first. So, there will be no conflict at this point, unless there is a conflicting driver claiming the same NVDIMM range. DAX then uses the pmem driver (or other byte-addressable driver) to mount a file system and creates a separate user-space mapping for mmap(). So, a (silent) map-type conflict will happen at this point, which may not be protected by the ioremap itself. > > DAX uses vm_insert_mixed(), which does not even check the existing map > > type to the physical address. > > Right, I think that's a problem... > > >> A general quibble separate from this patch is that we don't have a way > >> of knowing if ioremap() will reject or change our requested memory > >> type. Shouldn't the driver be explicitly requesting a known valid > >> type in advance? > > > > I agree we need a solution here. > > > >> Lastly we now have the PMEM API patches from Ross out for review where > >> he is assuming cached mappings with non-temporal writes: > >> https://lists.01.org/pipermail/linux-nvdimm/2015-May/000929.html. > >> This gives us WC semantics on writes which I believe has the nice > >> property of reducing the number of write transactions to memory. > >> Also, the numbers in the paper seem to be assuming DAX operation, but > >> this ioremap_wt() is in the driver and typically behind a file system. > >> Are the numbers relevant to that usage mode? > > > > I have not looked into the Ross's changes yet, but they do not seem to > > replace the use of ioremap_nocache(). If his changes can use WB type > > reliably, yes, we do not need a temporary solution of using ioremap_wt() > > in this driver. > > Hmm, yes you're right, it seems those patches did not change the > implementation to use ioremap_cache()... which happens to not be > implemented on all architectures. I'll take a look. Thanks, -Toshi -- 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>