On Thu 27-07-17 19:05:24, Andy Lutomirski wrote: > On Thu, Jul 27, 2017 at 2:57 PM, Ross Zwisler > <ross.zwisler@xxxxxxxxxxxxxxx> wrote: > > On Thu, Jul 27, 2017 at 10:09:07AM -0400, Jeff Moyer wrote: > >> Jan Kara <jack@xxxxxxx> writes: > >> > >> Hi, Jan, > >> > >> Thanks for looking into this! > >> > >> > There are couple of open questions with this implementation: > >> > > >> > 1) Is it worth the hassle? > >> > 2) Is S_SYNC good flag to use or should we use a new inode flag? > >> > 3) VM_FAULT_RO and especially passing of resulting 'pfn' from > >> > dax_iomap_fault() through filesystem fault handler to dax_pfn_mkwrite() in > >> > vmf->orig_pte is a bit of a hack. So far I'm not sure how to refactor > >> > things to make this cleaner. > >> > >> 4) How does an application discover that it is safe to flush from > >> userspace? > > > > I think that we would be best off with a new flag available via > > lsattr(1)/chattr(1). This would have the following advantages: > > > > 1) We could only set the flag if the inode supported DAX (either via the mount > > option or via the individual DAX flag). This would give NVML et al. one > > central way to detect whether it was safe to flush from userspace because the > > FS supported synchronous faults. > > > > 2) Defining a new flag prevents any confusion about whether the kernel version > > you have supports sync faults. Otherwise NVML would have to do something like > > look at the trio of (kernel version, S_SYNC flag, mount/inode option for DAX) > > which is complex and of course breaks for OS kernel versions. > > > > 3) Defining the flag in a generic way via lsattr/chattr opens the door for the > > same API and flag to be used by other filesystems in the future. > > I would advocate using a new fcntl() instead of lsattr for the > following reason: ISTM the fact that it's an *inode* flag in this > patchset is a bit of an implementation detail. I can easily imagine a > future implementation that makes it per-struct-file instead. A > fcntl() that asks "can I flush from userspace" would still work under > than scenario. Well, you are right I can make the implementation work with struct file flag as well - let's call it O_DAXDSYNC. However there are filesystem operations where you may need to answer question: Is there any fd with O_DAXDSYNC open against this inode (for operations that change file offset -> block mapping)? And in that case inode flag is straightforward while file flag is a bit awkward (you need to implement counter of fd's with that flag in the inode). Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
