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 >> >> write back the contents of the CPU caches in case of a crash. >> >> >> >> This patch changes to use ioremap_wt(), which provides uncached >> >> writes but cached reads, for improving read performance. >> >> >> >> Signed-off-by: Toshi Kani <toshi.kani@xxxxxx> >> >> --- >> >> drivers/block/pmem.c | 4 ++-- >> >> 1 file changed, 2 insertions(+), 2 deletions(-) >> >> >> >> diff --git a/drivers/block/pmem.c b/drivers/block/pmem.c >> >> index eabf4a8..095dfaa 100644 >> >> --- a/drivers/block/pmem.c >> >> +++ b/drivers/block/pmem.c >> >> @@ -139,11 +139,11 @@ static struct pmem_device *pmem_alloc(struct device *dev, struct resource *res) >> >> } >> >> >> >> /* >> >> - * Map the memory as non-cachable, as we can't write back the contents >> >> + * Map the memory as write-through, as we can't write back the contents >> >> * of the CPU caches in case of a crash. >> >> */ >> >> err = -ENOMEM; >> >> - 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. > 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. -- 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>