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

Christian Brauner
2023-08-10T08:20:17Z

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.