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.
