On Mon, Mar 16, 2020 at 10:55:09AM +0100, Christoph Hellwig wrote: > On Mon, Mar 16, 2020 at 10:52:24AM +0100, Jan Kara wrote: > > > This sounds reasonable to me. > > > > > > As for deprecating the mount option, I think at a minimum it needs to > > > continue be accepted as an option even if it is ignored to not break > > > existing setups. > > > > Agreed. But that's how we usually deprecate mount options. Also I'd say > > that statx() support for reporting DAX state and some education of > > programmers using DAX is required before we deprecate the mount option > > since currently applications check 'dax' mount option to determine how much > > memory they need to set aside for page cache before they consume everything > > else on the machine... > > I don't even think we should deprecate it. It isn't painful to maintain > and actually useful for testing. Instead we should expand it into a > tristate: > > dax=off > dax=flag > dax=always > > where the existing "dax" option maps to "dax=always" and nodax maps > to "dax=off". and dax=flag becomes the default for DAX capable devices. That works for me. In summary: - Applications must call statx to discover the current S_DAX state. - There exists an advisory file inode flag FS_XFLAG_DAX that can be changed on files that have no blocks allocated to them. Changing this flag does not necessarily change the S_DAX state immediately but programs can query the S_DAX state via statx. If FS_XFLAG_DAX is set and the fs is on pmem then it will always enable S_DAX at inode load time; if FS_XFLAG_DAX is not set, it will never enable S_DAX. Unless overridden... - There exists a dax= mount option. dax=off means "never set S_DAX, ignore FS_XFLAG_DAX"; dax=always means "always set S_DAX (at least on pmem), ignore FS_XFLAG_DAX"; and dax=iflag means "follow FS_XFLAG_DAX" and is the default. "dax" by itself means "dax=always". "nodax" means "dax=off". - There exists an advisory directory inode flag FS_XFLAG_DAX that can be changed at any time. The flag state is copied into any files or subdirectories created within that directory. If programs require that file access runs in S_DAX mode, they'll have to create those files themselves inside a directory with FS_XFLAG_DAX set, or mount the fs with dax=always. Ok? Let's please get this part finished for 5.8, then we can get back to arguing about fs-rmap and reflink and dax and whatnot. --D
