From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f41.google.com (mail-pa0-f41.google.com [209.85.220.41]) by kanga.kvack.org (Postfix) with ESMTP id 6873E6B009F for ; Fri, 29 May 2015 11:22:41 -0400 (EDT) Received: by pabru16 with SMTP id ru16so62997153pab.1 for ; Fri, 29 May 2015 08:22:41 -0700 (PDT) Received: from g2t2353.austin.hp.com (g2t2353.austin.hp.com. [15.217.128.52]) by mx.google.com with ESMTPS id fk17si8887606pdb.56.2015.05.29.08.22.40 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 29 May 2015 08:22:40 -0700 (PDT) Message-ID: <1432911782.23540.55.camel@misato.fc.hp.com> Subject: Re: [PATCH v10 12/12] drivers/block/pmem: Map NVDIMM with ioremap_wt() From: Toshi Kani Date: Fri, 29 May 2015 09:03:02 -0600 In-Reply-To: References: <1432739944-22633-1-git-send-email-toshi.kani@hp.com> <1432739944-22633-13-git-send-email-toshi.kani@hp.com> <20150529091129.GC31435@pd.tnic> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Dan Williams Cc: Borislav Petkov , Ross Zwisler , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , Andrew Morton , Arnd Bergmann , linux-mm@kvack.org, "linux-kernel@vger.kernel.org" , X86 ML , "linux-nvdimm@lists.01.org" , jgross@suse.com, Stefan Bader , Andy Lutomirski , hmh@hmh.eng.br, yigal@plexistor.com, Konrad Rzeszutek Wilk , "Elliott, Robert (Server Storage)" , mcgrof@suse.com, Christoph Hellwig , Matthew Wilcox On Fri, 2015-05-29 at 07:43 -0700, Dan Williams wrote: > On Fri, May 29, 2015 at 2:11 AM, Borislav Petkov 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 > >> --- > >> 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@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org