On Thu 09-04-20 09:59:27, Darrick J. Wong wrote: > And today: > > 1. There exists an in-kernel access mode flag S_DAX that is set when > file accesses go directly to persistent memory, bypassing the page > cache. Applications must call statx to discover the current S_DAX > state (STATX_ATTR_DAX). > > 2. There exists an advisory file inode flag FS_XFLAG_DAX that is > inherited from the parent directory FS_XFLAG_DAX inode flag at file > creation time. This advisory flag can be set or cleared at any > time, but doing so does not immediately affect the S_DAX state. > > Unless overridden by mount options (see (3)), if FS_XFLAG_DAX is set > and the fs is on pmem then it will enable S_DAX at inode load time; > if FS_XFLAG_DAX is not set, it will not enable S_DAX. > > 3. There exists a dax= mount option. > > "-o dax=never" means "never set S_DAX, ignore FS_XFLAG_DAX." > > "-o dax=always" means "always set S_DAX (at least on pmem), > and ignore FS_XFLAG_DAX." > > "-o dax" is an alias for "dax=always". > > "-o dax=inode" means "follow FS_XFLAG_DAX" and is the default. > > 4. There exists an advisory directory inode flag FS_XFLAG_DAX that can > be set or cleared at any time. The flag state is copied into any > files or subdirectories when they are created within that directory. > > 5. Programs that require a specific file access mode (DAX or not DAX) > can do one of the following: > > (a) Create files in directories that the FS_XFLAG_DAX flag set as > needed; or > > (b) Have the administrator set an override via mount option; or > > (c) Set or clear the file's FS_XFLAG_DAX flag as needed. Programs > must then cause the kernel to evict the inode from memory. This > can be done by: > > i> Closing the file and re-opening the file and using statx to > see if the fs has changed the S_DAX flag; and > > ii> If the file still does not have the desired S_DAX access > mode, either unmount and remount the filesystem, or close > the file and use drop_caches. > > 6. It's not unreasonable that users who want to squeeze every last bit > of performance out of the particular rough and tumble bits of their > storage also be exposed to the difficulties of what happens when the > operating system can't totally virtualize those hardware > capabilities. Your high performance sports car is not a Toyota > minivan, as it were. > > Given our overnight discussions, I don't think it'll be difficult to > hoist XFS_IDONTCACHE to the VFS so that 5.c.i is enough to change the > S_DAX state if nobody else is using the file. I still find the "S_DAX changes on inode eviction" confusing but I guess it's as good as it gets and with XFS_IDONTCACHE lifted to VFS, it seems acceptable so OK from me. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
