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

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Date:   Mon, 2 Jul 2018 09:34:09 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Lukas Czerner <lczerner@...hat.com>, Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@....de>, linux-nvdimm@...ts.01.org,
        Jeff Moyer <jmoyer@...hat.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2 2/2] ext4: handle layout changes to pinned DAX mappings

On Fri 29-06-18 09:13:00, Ross Zwisler wrote:
> 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@...ux.intel.com>
> > > Reviewed-by: Jan Kara <jack@...e.cz>
> > > ---
> > >  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.

I also don't see a way how ext4_insert_range() not calling
ext4_break_layouts() could lead to corruption. However the whole operation
with splitting (and possible zeroing out) of extents, truncation of extents
beyond EOF, etc. is complex enough that I'm not sure I've thought through
all the corner cases :-) So it at least deserves a comment why
ext4_break_layouts() is not necessary here (also as a reminder in case we
change the implementation and it would be suddently needed).

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