linux-ext4 - Re: [PATCH V5 00/12] Enable per-file/per-directory DAX operations V5

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Date:   Mon, 6 Apr 2020 12:00:05 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dave Chinner <david@...morbit.com>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [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.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR