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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 25 Nov 2020 21:44:18 +0100
Message-ID: <CAJfpegtL=T1mc6TDZdM1PaTUwv9Z8UyR8p171TK0d-0VS_E1hQ@mail.gmail.com>
Subject: Re: UAPI value collision: STATX_ATTR_MOUNT_ROOT vs STATX_ATTR_DAX
To:     Eric Sandeen <sandeen@redhat.com>
Cc:     David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Ira Weiny <ira.weiny@intel.com>, linux-fsdevel@vger.kernel.org,
        linux-man <linux-man@vger.kernel.org>,
        linux-kernel@vger.kernel.org
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> Related to this, nothing sets STATX_ATTR_DAX into statx->attributes_mask,
> anywhere in the kernel.
>
> The flag is set into statx->attributes in vfs_getattr_nosec(), but that
> does not know whether the particular filesystem under query supports dax
> or not.
>
> This is related to my other email about exactly what attributes_mask
> means, so should STATX_ATTR_DAX be set in statx->attributes_mask only
> in the filesystems that support dax?
>
> (And should that be done only if CONFIG_DAX is turned on, etc?)

I think the mask/flag combinations should mean:

mask:off, flag:off -> value unknown
mask:off, flag:on -> N/A
mask:on, flag:off -> value is off
mask:on, flag:on -> value is on

So mask should be off iff STATX_ATTR_DAX will not be set even if DAX
is enabled on a particular inode (such as on an old kernel not
supporting STATX_ATTR_DAX).

Thanks,
Miklos