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    SubjectRe: [PATCH V5 00/12] Enable per-file/per-directory DAX operations V5
    On Fri, Apr 03, 2020 at 11:18:43AM -0700, 'Ira Weiny' wrote:
    [snip]

    >
    > Ok For 5.8 why don't we not allow FS_XFLAG_DAX to be changed on files _at_
    > _all_...

    To be fair, I believe Christoph advocated for this at one time or another...

    Ira

    >
    > In summary:
    >
    > - Applications must call statx to discover the current S_DAX state.
    >
    > - There exists an advisory file inode flag FS_XFLAG_DAX that is set based on
    > the parent directory FS_XFLAG_DAX inode flag. (There is no way to change
    > this flag after file creation.)
    >
    > 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.
    > Unless overridden...
    >
    > - There exists a dax= mount option.
    >
    > "-o dax=off" means "never set S_DAX, ignore FS_XFLAG_DAX"
    > "-o nodax" means "dax=off"
    > "-o dax=always" means "always set S_DAX (at least on pmem), ignore FS_XFLAG_DAX"
    > "-o dax" by itself means "dax=always"
    > "-o dax=iflag" means "follow FS_XFLAG_DAX" and is the default
    >
    > - 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 when they are created within that directory. If programs
    > require file access runs in S_DAX mode, they'll have to create those files
    > inside a directory with FS_XFLAG_DAX set, or mount the fs with an
    > appropriate dax mount option.
    >
    >
    > ???
    >
    > Ira
    >

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