Re: [PATCH V6 6/8] fs/xfs: Combine xfs_diflags_to_linux() and xfs_diflags_to_iflags()

Jan Kara
2020-04-09T17:17:39Z

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@suse.com>
SUSE Labs, CR