On Mon, Sep 12, 2016 at 11:40:35AM +1000, Dave Chinner wrote: > On Thu, Sep 08, 2016 at 04:56:36PM -0600, Ross Zwisler wrote: > > On Wed, Sep 07, 2016 at 09:32:36PM -0700, Dan Williams wrote: > > > My understanding is that it is looking for the VM_MIXEDMAP flag which > > > is already ambiguous for determining if DAX is enabled even if this > > > dynamic listing issue is fixed. XFS has arranged for DAX to be a > > > per-inode capability and has an XFS-specific inode flag. We can make > > > that a common inode flag, but it seems we should have a way to > > > interrogate the mapping itself in the case where the inode is unknown > > > or unavailable. I'm thinking extensions to mincore to have flags for > > > DAX and possibly whether the page is part of a pte, pmd, or pud > > > mapping. Just floating that idea before starting to look into the > > > implementation, comments or other ideas welcome... > > > > I think this goes back to our previous discussion about support for the PMEM > > programming model. Really I think what NVML needs isn't a way to tell if it > > is getting a DAX mapping, but whether it is getting a DAX mapping on a > > filesystem that fully supports the PMEM programming model. This of course is > > defined to be a filesystem where it can do all of its flushes from userspace > > safely and never call fsync/msync, and that allocations that happen in page > > faults will be synchronized to media before the page fault completes. > > > > IIUC this is what NVML needs - a way to decide "do I use fsync/msync for > > everything or can I rely fully on flushes from userspace?" > > "need fsync/msync" is a dynamic state of an inode, not a static > property. i.e. users can do things that change an inode behind the > back of a mapping, even if they are not aware that this might > happen. As such, a filesystem can invalidate an existing mapping > at any time and userspace won't notice because it will simply fault > in a new mapping on the next access... > > > For all existing implementations, I think the answer is "you need to use > > fsync/msync" because we don't yet have proper support for the PMEM programming > > model. > > Yes, that is correct. > > FWIW, I don't think it will ever be possible to support this .... > wonderful "PMEM programming model" from any current or future kernel > filesystem without a very specific set of restrictions on what can > be done to a file. e.g. > > 1. the file has to be fully allocated and zeroed before > use. Preallocation/zeroing via unwritten extents is not > allowed. Sparse files are not allowed. Shared extents are > not allowed. > 2. set the "PMEM_IMMUTABLE" inode flag - filesystem must > check the file is fully allocated before allowing it to > be set, and caller must have CAP_LINUX_IMMUTABLE. > 3. Inode metadata is now immutable, and file data can only > be accessed and/or modified via mmap(). > 4. All non-mmap methods of inode data modification > will now fail with EPERM. > 5. all methods of inode metadata modification will now fail > with EPERM, timestamp udpdates will be ignored. > 6. PMEM_IMMUTABLE flag can only be removed if the file is > not currently mapped and caller has CAP_LINUX_IMMUTABLE. > > A flag like this /should/ make it possible to avoid fsync/msync() on > a file for existing filesystems, but it also means that such files > have significant management issues (hence the need for > CAP_LINUX_IMMUTABLE to cover it's use). Hmmm... I started to ponder such a flag, but ran into some questions. If it's PMEM_IMMUTABLE, does this mean that none of 1-6 apply if the filesystem discovers it isn't on pmem? I thought about just having a 'immutable metadata' flag where any timestamp, xattr, or block mapping update just returns EPERM. There wouldn't be any checks as in (1); if you left a hole in the file prior to setting the flag then you won't be filling it unless you clear the flag. OTOH if it merely made the metadata unchangeable then it's a stretch to get to non-mmap data accesses also being disallowed. Maybe the immutable metadata and mmap-only properties would only be implied if both DAX and IMMUTABLE_META are set on a file? Ok no more rambling until sleep. :) --D > > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx > -- > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
