Re: [PATCH 07/13] xfs: document the invalidate_bdev call in invalidate_bdev

[Christian Brauner <brauner@kernel.org>]

On Wed, Aug 09, 2023 at 03:39:23PM -0700, Darrick J. Wong wrote:
> On Wed, Aug 09, 2023 at 03:05:39PM -0700, Christoph Hellwig wrote:
> > Copy and paste the commit message from Darrick into a comment to explain
> > the seemly odd invalidate_bdev in xfs_shutdown_devices.
> 
>       ^ seemingly?
> > 
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > ---
> >  fs/xfs/xfs_super.c | 26 ++++++++++++++++++++++++++
> >  1 file changed, 26 insertions(+)
> > 
> > diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> > index 4ae3b01ed038c7..c169beb0d8cab3 100644
> > --- a/fs/xfs/xfs_super.c
> > +++ b/fs/xfs/xfs_super.c
> > @@ -399,6 +399,32 @@ STATIC void
> >  xfs_shutdown_devices(
> >  	struct xfs_mount	*mp)
> >  {
> > +	/*
> > +	 * Udev is triggered whenever anyone closes a block device or unmounts
> > +	 * a file systemm on a block device.
> > +	 * The default udev rules invoke blkid to read the fs super and create
> > +	 * symlinks to the bdev under /dev/disk.  For this, it uses buffered
> > +	 * reads through the page cache.
> > +	 *
> > +	 * xfs_db also uses buffered reads to examine metadata.  There is no
> > +	 * coordination between xfs_db and udev, which means that they can run
> > +	 * concurrently.  Note there is no coordination between the kernel and
> > +	 * blkid either.
> > +	 *
> > +	 * On a system with 64k pages, the page cache can cache the superblock
> > +	 * and the root inode (and hence the root directory) with the same 64k
> > +	 * page.  If udev spawns blkid after the mkfs and the system is busy
> > +	 * enough that it is still running when xfs_db starts up, they'll both
> > +	 * read from the same page in the pagecache.
> > +	 *
> > +	 * The unmount writes updated inode metadata to disk directly.  The XFS
> > +	 * buffer cache does not use the bdev pagecache, nor does it invalidate
> > +	 * the pagecache on umount.  If the above scenario occurs, the pagecache
> 
> This sentence reads a little strangely, since "nor does it invalidate"
> would seem to conflict with the invalidate_bdev call below.  I suggest
> changing the verb a bit:
> 
> "The XFS buffer cache does not use the bdev pagecache, so it needs to
> invalidate that pagecache on unmount."
> 
> With those two things changed,
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Fixed in-tree.