From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH v6 3/5] mm: introduce mmap3 for safely defining new mmap flags Date: Fri, 25 Aug 2017 06:00:11 -0700 Message-ID: <20170825130011.GA30072@infradead.org> References: <150353211413.5039.5228914877418362329.stgit@dwillia2-desk3.amr.corp.intel.com> <150353213097.5039.6729469069608762658.stgit@dwillia2-desk3.amr.corp.intel.com> <20170824165546.GA3121@infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-nvdimm-bounces-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org Sender: "Linux-nvdimm" To: Dan Williams Cc: linux-xfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Jan Kara , Arnd Bergmann , "linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org" , Linux API , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , Christoph Hellwig , Linux MM , Andy Lutomirski , linux-fsdevel , Andrew Morton , "Kirill A. Shutemov" List-Id: linux-api@vger.kernel.org On Thu, Aug 24, 2017 at 10:36:02AM -0700, Dan Williams wrote: > I'll let Andy and Kirill restate their concerns, but one of the > arguments that swayed me is that any new mmap flag with this hack must > be documented to only work with MAP_SHARED and that MAP_PRIVATE is > silently ignored. I agree with the mess and delays it causes for other > archs and libc, but at the same time this is for new applications and > libraries that know to look for the new flag, so they need to do the > extra work to check for the new syscall. True. That is for the original hack, but I spent some more time looking at the mmap code, and there is one thing I noticed: include/uapi/asm-generic/mman-common.h: #define MAP_SHARED 0x01 /* Share changes */ #define MAP_PRIVATE 0x02 /* Changes are private */ #define MAP_TYPE 0x0f /* Mask for type of mapping */ mm/mmap.c: if (file) { struct inode *inode = file_inode(file); switch (flags & MAP_TYPE) { case MAP_SHARED: ... case MAP_PRIVATE: ... default: return -EINVAL; } and very similar for the anonymous and nommu cases. So if we pick e.g. 0x4 as the valid bit we don't even need to overload the MAP_SHARED and MAP_PRIVATE meaning. > > However, if the fcntl lease approach works for the DMA cases then we > only have the one mmap flag to add for now, so maybe the weird > MAP_{SHARED|PRIVATE} semantics are tolerable. ---end quoted text---