Re: [Q] gfs2: mmap write vs. punch_hole consistency

[Dave Chinner <david@fromorbit.com>]

On Fri, Sep 06, 2019 at 10:52:41PM +0200, Andreas Gruenbacher wrote:
> Hi,
> 
> I've just fixed a mmap write vs. truncate consistency issue on gfs on
> filesystems with a block size smaller that the page size [1].
> 
> It turns out that the same problem exists between mmap write and hole
> punching, and since xfstests doesn't seem to cover that,

AFAIA, fsx exercises it pretty often. Certainly it's found problems
with XFS in the past w.r.t. these operations.

> I've written a
> new test [2].

I suspect that what we really want is a test that runs
_test_generic_punch using mmap rather than pwrite...

> Ext4 and xfs both pass that test; they both apparently
> mark the pages that have a hole punched in them as read-only so that
> page_mkwrite is called before those pages can be written to again.

XFS invalidates the range being hole punched (see
xfs_flush_unmap_range() under XFS_MMAPLOCK_EXCL, which means any
attempt to fault that page back in will block on the MMAPLOCK until
the hole punch finishes.

> gfs2 fails that: for some reason, the partially block-mapped pages are
> not marked read-only on gfs2, and so page_mkwrite is not called for the
> partially block-mapped pages, and the hole is not filled in correctly.
> 
> The attached patch fixes the problem, but this really doesn't look right
> as neither ext4 nor xfs require this kind of hack.  So what am I
> overlooking, how does this work on ext4 and xfs?

XFS uses XFS_MMAPLOCK_* to serialise page faults against extent
manipulations (shift, hole punch, truncate, swap, etc) and ext4 uses
a similar locking mechanism to do the same thing. Fundamentally, the
page cache does not provide the necessary mechanisms to detect and
prevent invalidation races inside EOF....

> 
> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
> ---
>  fs/gfs2/bmap.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
> index 9ef543dd38e2..e677e813be4c 100644
> --- a/fs/gfs2/bmap.c
> +++ b/fs/gfs2/bmap.c
> @@ -2475,6 +2475,13 @@ int __gfs2_punch_hole(struct file *file, loff_t offset, loff_t length)
>  			if (error)
>  				goto out;
>  		}
> +		/*
> +		 * If the first or last page partially lies in the hole, mark
> +		 * the page read-only so that memory-mapped writes will trigger
> +		 * page_mkwrite.
> +		 */
> +		pagecache_isize_extended(inode, offset, inode->i_size);
> +		pagecache_isize_extended(inode, offset + length, inode->i_size);

See xfs_flush_unmap_range(), which is run under XFS_MMAPLOCK_EXCL
to serialise against incoming page faults...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com