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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 1 Dec 2020 12:04:56 -0800
Message-ID: <CAHk-=wgOu9vgUfOSsjO3hHHxGDn4BKhitC_8XCfgmGKiiSm_ag@mail.gmail.com>
Subject: Re: [PATCH 2/2] statx: move STATX_ATTR_DAX attribute handling to filesystems
To:     Eric Sandeen <sandeen@redhat.com>
Cc:     Miklos Szeredi <mszeredi@redhat.com>,
        Ira Weiny <ira.weiny@intel.com>,
        David Howells <dhowells@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-man <linux-man@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        xfs <linux-xfs@vger.kernel.org>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Xiaoli Feng <xifeng@redhat.com>
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On Tue, Dec 1, 2020 at 8:59 AM Eric Sandeen <sandeen@redhat.com> 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