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

[Jan Kara <jack@suse.cz>]

On Fri 06-09-19 23:48:31, Andreas Gruenbacher wrote:
> On Fri, Sep 6, 2019 at 11:28 PM Dave Chinner <david@fromorbit.com> wrote:
> > 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.
> 
> This isn't about writes during the hole punching, this is about writes
> once the hole is punched. For example, the test case I've posted
> creates the following file layout with 1k blocksize:
> 
>   DDDD DDDD DDDD
> 
> Then it punches a hole like this:
> 
>   DDHH HHHH HHDD
> 
> Then it fills the hole again with mwrite:
> 
>   DDDD DDDD DDDD
> 
> As far as I can tell, that needs to trigger page faults on all three
> pages. I did get these on ext4; judging from the fact that xfs works,
> the also seem to occur there; but on gfs2, page_mkwrite isn't called
> for the two partially mapped pages, only for the page in the middle
> that's entirely within the hole. And I don't see where those pages are
> marked read-only; it appears like pagecache_isize_extended isn't
> called on ext4 or xfs. So how does this work there?

The trick ext4 & xfs use is that they writeout the range being punched
first (see e.g. ext4_punch_hole() calling filemap_write_and_wait_range() or
xfs_flush_unmap_range() called from xfs_free_file_space()). This writeout
also has the effect that all the page mappings for that range get
write-protected.

Another related issue is what Dave points out: Even if you use writeout to
writeprotect pages, GFS2 still seems to have a race where page fault can
come while you are freeing blocks and if you allow that you usually get
into a problematic state. Effects depend on fs implementation details but
usually it can result in stale data exposure or fs corruption.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR