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. DAX uses vm_insert_mixed(), which does not even check the existing map type to the physical address. > 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. 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>