Re: [PATCH v2 2/2] ext4: handle layout changes to pinned DAX mappings

[Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Fri, Jun 29, 2018 at 02:02:23PM +0200, Lukas Czerner wrote:
> On Wed, Jun 27, 2018 at 03:22:52PM -0600, Ross Zwisler wrote:
> > Follow the lead of xfs_break_dax_layouts() and add synchronization between
> > operations in ext4 which remove blocks from an inode (hole punch, truncate
> > down, etc.) and pages which are pinned due to DAX DMA operations.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
> > Reviewed-by: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>
> > ---
> >  fs/ext4/ext4.h     |  1 +
> >  fs/ext4/extents.c  | 12 ++++++++++++
> >  fs/ext4/inode.c    | 46 ++++++++++++++++++++++++++++++++++++++++++++++
> >  fs/ext4/truncate.h |  4 ++++
> >  4 files changed, 63 insertions(+)
> > 
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index 0b127853c584..34bccd64d83d 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -2460,6 +2460,7 @@ extern int ext4_get_inode_loc(struct inode *, struct ext4_iloc *);
> >  extern int ext4_inode_attach_jinode(struct inode *inode);
> >  extern int ext4_can_truncate(struct inode *inode);
> >  extern int ext4_truncate(struct inode *);
> > +extern int ext4_break_layouts(struct inode *);
> >  extern int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length);
> >  extern int ext4_truncate_restart_trans(handle_t *, struct inode *, int nblocks);
> >  extern void ext4_set_inode_flags(struct inode *);
> > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> > index 0057fe3f248d..a6aef06f455b 100644
> > --- a/fs/ext4/extents.c
> > +++ b/fs/ext4/extents.c
> > @@ -4820,6 +4820,13 @@ static long ext4_zero_range(struct file *file, loff_t offset,
> >  		 * released from page cache.
> >  		 */
> >  		down_write(&EXT4_I(inode)->i_mmap_sem);
> > +
> > +		ret = ext4_break_layouts(inode);
> > +		if (ret) {
> > +			up_write(&EXT4_I(inode)->i_mmap_sem);
> > +			goto out_mutex;
> > +		}
> > +
> >  		ret = ext4_update_disksize_before_punch(inode, offset, len);
> >  		if (ret) {
> >  			up_write(&EXT4_I(inode)->i_mmap_sem);
> > @@ -5493,6 +5500,11 @@ int ext4_collapse_range(struct inode *inode, loff_t offset, loff_t len)
> >  	 * page cache.
> >  	 */
> >  	down_write(&EXT4_I(inode)->i_mmap_sem);
> > +
> > +	ret = ext4_break_layouts(inode);
> > +	if (ret)
> > +		goto out_mmap;
> 
> Hi,
> 
> don't we need to do the same for ext4_insert_range() since we're about
> to truncate_pagecache() as well ?
> 
> /thinking out loud/
> Xfs seems to do this before every fallocate operation, but in ext4
> it does not seem to be needed at least for simply allocating falocate...

I saw the case in ext4_insert_range(), and decided that we didn't need to
worry about synchronizing with DAX because no blocks were being removed from
the inode's extent map.  IIUC the truncate_pagecache() call is needed because
we are unmapping and removing any page cache mappings for the part of the file
after the insert because those blocks are now at a different offset in the
inode.  Because at the end of the operation we haven't removed any DAX pages
from the inode, we have nothing that we need to synchronize.

Hmm, unless this is a failure case we care about fixing?
 1) schedule I/O via O_DIRECT to page X
 2) fallocate(FALLOC_FL_INSERT_RANGE) to block < X, shifting X to a larger
    offset
 3) O_DIRECT I/O from 1) completes, but ends up writing into the *new* block
    that resides at X - the I/O from 1) completes

In this case the user is running I/O and issuing the fallocate at the same
time, and the sequencing could have worked out that #1 and #2 were reversed,
giving you the same behavior.  IMO this seems fine and that we shouldn't have
the DAX synchronization call in ext4_insert_range(), but I'm happy to add it
if I'm wrong.