From: Ross Zwisler Subject: Re: [PATCH v4 1/5] mm: add mkwrite param to vm_insert_mixed() Date: Mon, 24 Jul 2017 09:13:03 -0600 Message-ID: <20170724151303.GA1639@linux.intel.com> References: <20170721223956.29485-1-ross.zwisler@linux.intel.com> <20170721223956.29485-2-ross.zwisler@linux.intel.com> <20170724111531.GG652@quack2.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: linux-doc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, David Airlie , Dave Chinner , dri-devel-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org, linux-mm , Andreas Dilger , Krzysztof Kozlowski , Christoph Hellwig , Jonathan Corbet , linux-samsung-soc , Joonyoung Shim , "Darrick J. Wong" , Rob Clark , Patrik Jakobsson , Tomi Valkeinen , ext4 hackers , linux-arm-msm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Steven Rostedt , "linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org" , Alexander Viro , Ingo Molnar , "linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org" Return-path: Content-Disposition: inline In-Reply-To: <20170724111531.GG652-4I4JzKEfoa/jFM9bn6wA6Q@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-nvdimm-bounces-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org Sender: "Linux-nvdimm" List-Id: linux-ext4.vger.kernel.org On Mon, Jul 24, 2017 at 01:15:31PM +0200, Jan Kara wrote: > On Sat 22-07-17 09:21:31, Dan Williams wrote: > > On Fri, Jul 21, 2017 at 3:39 PM, 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 the dax_pfn_mkwrite() helper. We will instead use > > > dax_iomap_fault() to handle write-protection faults. > > > > > > 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 > > > --- > > > drivers/dax/device.c | 2 +- > > > drivers/gpu/drm/exynos/exynos_drm_gem.c | 3 ++- > > > drivers/gpu/drm/gma500/framebuffer.c | 2 +- > > > drivers/gpu/drm/msm/msm_gem.c | 3 ++- > > > drivers/gpu/drm/omapdrm/omap_gem.c | 6 ++++-- > > > drivers/gpu/drm/ttm/ttm_bo_vm.c | 2 +- > > > fs/dax.c | 2 +- > > > include/linux/mm.h | 2 +- > > > mm/memory.c | 27 +++++++++++++++++++++------ > > > 9 files changed, 34 insertions(+), 15 deletions(-) > > > > > > diff --git a/drivers/dax/device.c b/drivers/dax/device.c > > > index e9f3b3e..3973521 100644 > > > --- a/drivers/dax/device.c > > > +++ b/drivers/dax/device.c > > > @@ -273,7 +273,7 @@ static int __dev_dax_pte_fault(struct dev_dax *dev_dax, struct vm_fault *vmf) > > > > > > pfn = phys_to_pfn_t(phys, dax_region->pfn_flags); > > > > > > - rc = vm_insert_mixed(vmf->vma, vmf->address, pfn); > > > + rc = vm_insert_mixed(vmf->vma, vmf->address, pfn, false); > > > > Ugh, I generally find bool flags unreadable. They place a tax on > > jumping to function definition to recall what true and false mean. If > > we want to go this 'add an argument' route can we at least add an enum > > like: > > > > enum { > > PTE_MKDIRTY, > > PTE_MKCLEAN, > > }; > > > > ...to differentiate the two cases? > > So how I usually deal with this is that I create e.g.: > > __vm_insert_mixed() that takes the bool argument, make vm_insert_mixed() > pass false, and vm_insert_mixed_mkwrite() pass true. That way there's no > code duplication, old call sites can stay unchanged, the naming clearly > says what's going on... Ah, that does seem cleaner. I'll try that for v5.