Re: [PATCH 2/2] statx: move STATX_ATTR_DAX attribute handling to filesystems

[Linus Torvalds]

On Tue, Dec 1, 2020 at 8:59 AM Eric Sandeen <sandeen@xxxxxxxxxx> wrote:
>
> It's a bit odd to set STATX_ATTR_DAX into the statx attributes in the VFS;
> while the VFS can detect the current DAX state, it is the filesystem which
> actually sets S_DAX on the inode, and the filesystem is the place that
> knows whether DAX is something that the "filesystem actually supports" [1]
> so that the statx attributes_mask can be properly set.
>
> So, move STATX_ATTR_DAX attribute setting to the individual dax-capable
> filesystems, and update the attributes_mask there as well.

I'm not really understanding the logic behind this.

The whole IS_DAX(inode) thing exists in various places outside the
low-level filesystem, why shouldn't stat() do this?

If IS_DAX() is incorrect, then we have much bigger problems than some
stat results. We have core functions like generic_file_read_iter() etc
all making actual behavioral judgements on IS_DAX().

And if IS_DAX() is correct, then why shouldn't this just be done in
generic code? Why move it to every individual filesystem?

               Linus