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SubjectRe: [PATCH V5 00/12] Enable per-file/per-directory DAX operations V5
On Fri 03-04-20 11:18:43, Ira Weiny wrote:
> Ok For 5.8 why don't we not allow FS_XFLAG_DAX to be changed on files _at_
> _all_...
> 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...

OK, after considering all the options we were hashing out here, I think
this is the best API. There isn't the confusing "S_DAX will magically
switch on inode eviction" and although the functionality is limited, I
think 90% of users would end up using the functionality like this anyway.

> - 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.


Jan Kara <>

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