Re: [PATCH 07/13] xfs: Convert to use invalidate_lock

[Jan Kara <jack@suse.cz>]

On Wed 26-05-21 07:40:41, Dave Chinner wrote:
> On Tue, May 25, 2021 at 03:50:44PM +0200, Jan Kara wrote:
> > Use invalidate_lock instead of XFS internal i_mmap_lock. The intended
> > purpose of invalidate_lock is exactly the same. Note that the locking in
> > __xfs_filemap_fault() slightly changes as filemap_fault() already takes
> > invalidate_lock.
> > 
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > CC: <linux-xfs@vger.kernel.org>
> > CC: "Darrick J. Wong" <darrick.wong@oracle.com>
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  fs/xfs/xfs_file.c  | 12 ++++++-----
> >  fs/xfs/xfs_inode.c | 52 ++++++++++++++++++++++++++--------------------
> >  fs/xfs/xfs_inode.h |  1 -
> >  fs/xfs/xfs_super.c |  2 --
> >  4 files changed, 36 insertions(+), 31 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > index 396ef36dcd0a..dc9cb5c20549 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -1282,7 +1282,7 @@ xfs_file_llseek(
> >   *
> >   * mmap_lock (MM)
> >   *   sb_start_pagefault(vfs, freeze)
> > - *     i_mmaplock (XFS - truncate serialisation)
> > + *     invalidate_lock (vfs/XFS_MMAPLOCK - truncate serialisation)
> >   *       page_lock (MM)
> >   *         i_lock (XFS - extent map serialisation)
> >   */
> > @@ -1303,24 +1303,26 @@ __xfs_filemap_fault(
> >  		file_update_time(vmf->vma->vm_file);
> >  	}
> >  
> > -	xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> >  	if (IS_DAX(inode)) {
> >  		pfn_t pfn;
> >  
> > +		xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> >  		ret = dax_iomap_fault(vmf, pe_size, &pfn, NULL,
> >  				(write_fault && !vmf->cow_page) ?
> >  				 &xfs_direct_write_iomap_ops :
> >  				 &xfs_read_iomap_ops);
> >  		if (ret & VM_FAULT_NEEDDSYNC)
> >  			ret = dax_finish_sync_fault(vmf, pe_size, pfn);
> > +		xfs_iunlock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> >  	} else {
> > -		if (write_fault)
> > +		if (write_fault) {
> > +			xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> >  			ret = iomap_page_mkwrite(vmf,
> >  					&xfs_buffered_write_iomap_ops);
> > -		else
> > +			xfs_iunlock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> > +		} else
> >  			ret = filemap_fault(vmf);
> >  	}
> > -	xfs_iunlock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> 
> This seems kinda messy. filemap_fault() basically takes the
> invalidate lock around the entire operation, it runs, so maybe it
> would be cleaner to implement it as:
> 
> filemap_fault_locked(vmf)
> {
> 	/* does the filemap fault work */
> }
> 
> filemap_fault(vmf)
> {
> 	filemap_invalidate_down_read(...)
> 	ret = filemap_fault_locked(vmf)
> 	filemap_invalidate_up_read(...)
> 	return ret;
> }
> 
> And that means XFS could just call filemap_fault_locked() and not 
> have to do all this messy locking just to avoid holding the lock
> that filemap_fault has now internalised.

Sure, I can do that.

> > @@ -355,8 +358,11 @@ xfs_isilocked(
> >  
> >  	if (lock_flags & (XFS_MMAPLOCK_EXCL|XFS_MMAPLOCK_SHARED)) {
> >  		if (!(lock_flags & XFS_MMAPLOCK_SHARED))
> > -			return !!ip->i_mmaplock.mr_writer;
> > -		return rwsem_is_locked(&ip->i_mmaplock.mr_lock);
> > +			return !debug_locks ||
> > +				lockdep_is_held_type(
> > +					&VFS_I(ip)->i_mapping->invalidate_lock,
> > +					0);
> > +		return rwsem_is_locked(&VFS_I(ip)->i_mapping->invalidate_lock);
> >  	}
> 
> <sigh>
> 
> And so here we are again, losing more of our read vs write debug
> checks on debug kernels when lockdep is not enabled....
> 
> Can we please add rwsem_is_locked_read() and rwsem_is_locked_write()
> wrappers that just look at the rwsem counter value to determine how
> the lock is held? Then the mrlock_t can go away entirely....

Apparently someone already did that for XFS as Darrick pointed out. So we
just have to sort out how to merge it.

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