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

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Date:   Wed, 27 Jun 2018 14:10:37 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>, linux-nvdimm@...ts.01.org,
        Jeff Moyer <jmoyer@...hat.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 2/2] ext4: handle layout changes to pinned DAX mappings

On Fri, Jun 22, 2018 at 10:19:15AM +0200, Jan Kara wrote:
> On Wed 20-06-18 16:15:03, 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>
> > ---
> >  fs/ext4/ext4.h     |  1 +
> >  fs/ext4/extents.c  | 12 ++++++++++++
> >  fs/ext4/inode.c    | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
> >  fs/ext4/truncate.h |  1 +
> >  4 files changed, 62 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;
> > +
> >  	/*
> >  	 * Need to round down offset to be aligned with page size boundary
> >  	 * for page size > block size.
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 2ea07efbe016..c795e5118745 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -4193,6 +4193,41 @@ int ext4_update_disksize_before_punch(struct inode *inode, loff_t offset,
> >  	return 0;
> >  }
> >  
> > +static void ext4_wait_dax_page(struct ext4_inode_info *ei, bool *did_unlock)
> > +{
> > +	*did_unlock = true;
> > +	up_write(&ei->i_mmap_sem);
> > +	schedule();
> > +	down_write(&ei->i_mmap_sem);
> > +}
> > +
> > +int ext4_break_layouts(struct inode *inode)
> > +{
> > +	struct ext4_inode_info *ei = EXT4_I(inode);
> > +	struct page *page;
> > +	bool retry;
> > +	int error;
> > +
> > +	if (unlikely(!rwsem_is_locked(&ei->i_mmap_sem))) {
> > +		WARN_ON_ONCE(1);
> > +		return -EINVAL;
> > +	}
> 
> This could be shortened as:
> 
> if (WARN_ON_ONCE(!rwsem_is_locked(&ei->i_mmap_sem))) {
> }
> 
> couldn't it?

Yep, that's much better.  Thank you for the review.

> Besides that the patch looks to me. You can add:
> 
> Reviewed-by: Jan Kara <jack@...e.cz>
> 
> And I'm really wondering which protection are we still missing that you are
> still able to hit the warning with these patches applied.

I'm also very curious and am going to dig into that this week.