On 26.08.2017 17:15, Dan Williams wrote: > On Sat, Aug 26, 2017 at 12:40 AM, Helge Deller <deller@xxxxxx> wrote: >> * Dan Williams <dan.j.williams@xxxxxxxxx>: >>> On Fri, Aug 25, 2017 at 9:19 AM, Helge Deller <deller@xxxxxx> wrote: >>>> On 25.08.2017 18:16, Kirill A. Shutemov wrote: >>>>> On Fri, Aug 25, 2017 at 09:02:36AM -0700, Christoph Hellwig wrote: >>>>>> On Fri, Aug 25, 2017 at 06:58:03PM +0300, Kirill A. Shutemov wrote: >>>>>>> Not all archs are ready for this: >>>>>>> >>>>>>> arch/parisc/include/uapi/asm/mman.h:#define MAP_TYPE 0x03 /* Mask for type of mapping */ >>>>>>> arch/parisc/include/uapi/asm/mman.h:#define MAP_FIXED 0x04 /* Interpret addr exactly */ >>>>>> >>>>>> I'd be happy to say that we should not care about parisc for >>>>>> persistent memory. We'll just have to find a way to exclude >>>>>> parisc without making life too ugly. >>>>> >>>>> I don't think creapling mmap() interface for one arch is the right way to >>>>> go. I think the interface should be universal. >>>>> >>>>> I may imagine MAP_DIRECT can be useful not only for persistent memory. >>>>> For tmpfs instead of mlock()? >>>> >>>> On parisc we have >>>> #define MAP_SHARED 0x01 /* Share changes */ >>>> #define MAP_PRIVATE 0x02 /* Changes are private */ >>>> #define MAP_TYPE 0x03 /* Mask for type of mapping */ >>>> #define MAP_FIXED 0x04 /* Interpret addr exactly */ >>>> #define MAP_ANONYMOUS 0x10 /* don't use a file */ >>>> >>>> So, if you need a MAP_DIRECT, wouldn't e.g. >>>> #define MAP_DIRECT 0x08 >>>> be possible (for parisc, and others 0x04). >>>> And if MAP_TYPE needs to include this flag on parisc: >>>> #define MAP_TYPE (0x03 | 0x08) /* Mask for type of mapping */ >>> >>> The problem here is that to support new the mmap flags the arch needs >>> to find a flag that is guaranteed to fail on older kernels. Defining >>> MAP_DIRECT to 0x8 on parisc doesn't work because it will simply be >>> ignored on older parisc kernels. >>> >>> However, it's already the case that several archs have their own >>> sys_mmap entry points. Those archs that can't follow the common scheme >>> (only parsic it seems) will need to add a new mmap syscall. I think >>> that's a reasonable tradeoff to allow every other architecture to add >>> this support with their existing mmap syscall paths. >> >> I don't want other architectures to suffer just because of parisc. >> But adding a new syscall just for usage on parisc won't work either, >> because nobody will add code to call it then. > > I don't understand this comment, if / when parisc gets around to > adding pmem and dax support why wouldn't libc grow support for the new > parisc mmap variant? Also, it's not just MAP_DIRECT you would also > need space for a MAP_SYNC flag. > >>> That means MAP_DIRECT should be defined to MAP_TYPE on parisc until it >>> later defines an opt-in mechanism to a new syscall that honors >>> MAP_DIRECT as a valid flag. >> >> I'd instead propose to to introduce an ABI breakage for parisc users >> (which aren't many). Most parisc users update their kernel regularily >> anyway, because we fixed so many bugs in the latest kernel. >> >> With the following patch pushed down to the stable kernel series, >> MAP_DIRECT will fail as expected on those kernels, while we can >> keep parisc up with current developments regarding MAP_DIRECT. > > The whole point is to avoid an ABI regression and the chance for false > positive results. We're immediately stuck if some application was > expecting 0x8 to be ignored, or conversely an application that > absolutely needs to rely on MAP_SYNC/MAP_DIRECT semantics assumes the > wrong result on a parisc kernel where they are ignored. > > I have not seen any patches for parisc pmem+dax enabling so it seems > too early to worry about these "last mile" enabling features of > MAP_DIRECT and MAP_SYNC. In particular parisc doesn't appear to have > ARCH_ENABLE_MEMORY_HOTPLUG, so as far as I can see it can't yet > support the ZONE_DEVICE scheme that is a pre-requisite for MAP_DIRECT. I see, but then it's probably best to not to define any MAP_DIRECT or MAP_SYNC at all in the headers of those arches which don't support pmem+dax (parisc, m68k, alpha, and probably quite some others). That way applications can detect at configure time if the platform supports that, and can leave out the functionality completely. Helge
