Re: [PATCH 08/12] iomap: zeroing already holds invalidate_lock

[Dave Chinner <david@fromorbit.com>]

On Wed, Sep 18, 2024 at 07:15:23AM +0200, Christoph Hellwig wrote:
> On Tue, Sep 17, 2024 at 02:29:35PM -0700, Darrick J. Wong wrote:
> > On Tue, Sep 10, 2024 at 07:39:10AM +0300, Christoph Hellwig wrote:
> > > All callers of iomap_zero_range already hold invalidate_lock, so we can't
> > > take it again in iomap_file_buffered_write_punch_delalloc.
> > > 
> > > Use the passed in flags argument to detect if we're called from a zeroing
> > > operation and don't take the lock again in this case.
> > > 
> > > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > > ---
> > >  fs/iomap/buffered-io.c | 10 ++++++++--
> > >  1 file changed, 8 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > > index 52f285ae4bddcb..3d7e69a542518a 100644
> > > --- a/fs/iomap/buffered-io.c
> > > +++ b/fs/iomap/buffered-io.c
> > > @@ -1188,8 +1188,13 @@ static void iomap_write_delalloc_release(struct inode *inode, loff_t start_byte,
> > >  	 * folios and dirtying them via ->page_mkwrite whilst we walk the
> > >  	 * cache and perform delalloc extent removal. Failing to do this can
> > >  	 * leave dirty pages with no space reservation in the cache.
> > > +	 *
> > > +	 * For zeroing operations the callers already hold invalidate_lock.
> > >  	 */
> > > -	filemap_invalidate_lock(inode->i_mapping);
> > > +	if (flags & IOMAP_ZERO)
> > > +		rwsem_assert_held_write(&inode->i_mapping->invalidate_lock);
> > 
> > Does the other iomap_zero_range user (gfs2) take the invalidate lock?
> > AFAICT it doesn't.  Shouldn't we annotate iomap_zero_range to say that
> > callers have to hold i_rwsem and the invalidate_lock?
> 
> gfs2 does not hold invalidate_lock over iomap_zero_range.  But
> it also does not use iomap_file_buffered_write_punch_delalloc at
> all, which is what requires the lock (and asserts that it is held).

Not a fan of this dichotomy.  It means that filesystems that don't
support IOMAP_DELALLOC don't need to hold the invalidate lock to
zero, but filesystems that do support IOMAP_DELALLOC do need to hold
it.

I'd kinda prefer there be one locking rule for all callers; it makes
it much easy to determine if the callers are doing the right thing
without needing to know if the filesystem is IOMAP_DELALLOC capable
or not. At minimum, it needs to be clearly documented.

-Dave.
-- 
Dave Chinner
david@fromorbit.com