linux-kernel - Re: [RFC 1/2] gfs2: replace sd_aspace with sd

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Message-ID: <CAHc6FU4J6MsEaUFUfp_ZpuYKyXRpZ=FTJE9T=iRQgbByQWZOFA@mail.gmail.com>
Date: Thu, 10 Apr 2025 21:20:03 +0200
From: Andreas Gruenbacher <agruenba@...hat.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: cgroups@...r.kernel.org, Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>, 
	Jan Kara <jack@...e.cz>, Rafael Aquini <aquini@...hat.com>, gfs2@...ts.linux.dev, 
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 1/2] gfs2: replace sd_aspace with sd_inode

On Thu, Apr 10, 2025 at 11:01 AM Christoph Hellwig <hch@...radead.org> wrote:
> On Mon, Apr 07, 2025 at 08:21:01PM +0200, Andreas Gruenbacher wrote:
> > Use a dummy inode as mapping->host of the address spaces for global as
> > well as per-inode metadata.  The global metadata address space is now
> > accessed as gfs2_aspace(sdp) instead of sdp->sd_aspace.  The per-inode
> > metadata address spaces are still accessed as
> > gfs2_glock2aspace(GFS2_I(inode)->i_gl).
> >
> > Based on a previous version from Bob Peterson from several years ago.
>
> Please explain why you are doing this, not just what.

Right, I have this description now:

    Currently, sdp->sd_aspace and the per-inode metadata address spaces use
    sb->s_bdev->bd_mapping->host as their ->host.  Folios in those address
    spaces will thus appear to be on "bdev" rather than on "gfs2"
    filesystems.  Those "bdev" filesystems will have the SB_I_CGROUPWB flag
    set to indicate cgroup writeback support.  In fact, gfs2 doesn't support
    cgroup writeback, though.

    To fix that, use a "dummy" gfs2 inode as ->host of those address spaces
    instead.  This will then allow functions like inode_to_wb() to determine
    that the folio belongs to a a filesystem without cgroup writeback
    support.


Thanks,
Andreas