Re: [PATCH 4/8] mm, dax: truncate dax mappings at bdev or fs shutdown

[Jan Kara <jack@suse.cz>]

On Tue 17-11-15 12:16:14, Dan Williams wrote:
> Currently dax mappings survive block_device shutdown.  While page cache
> pages are permitted to be read/written after the block_device is torn
> down this is not acceptable in the dax case as all media access must end
> when the device is disabled.  The pfn backing a dax mapping is permitted
> to be invalidated after bdev shutdown and this is indeed the case with
> brd.
> 
> When a dax capable block_device driver calls del_gendisk() in its
> shutdown path, or a filesystem evicts an inode it needs to ensure that
> all the pfns that had been mapped via bdev_direct_access() are unmapped.
> This is different than the pagecache backed case where
> truncate_inode_pages() is sufficient to end I/O to pages mapped to a
> dying inode.
> 
> Since dax bypasses the page cache we need to unmap in addition to
> truncating pages.  Also, since dax mappings are not accounted in the
> mapping radix we uncoditionally truncate all inodes with the S_DAX flag.
> Likely when we add support for dynamic dax enable/disable control we'll
> have infrastructure to detect if the inode is unmapped and can skip the
> truncate.
> 
> Cc: <stable@vger.kernel.org>
> Cc: Jan Kara <jack@suse.com>
> Cc: Dave Chinner <david@fromorbit.com>
> Cc: Matthew Wilcox <willy@linux.intel.com>
> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  fs/inode.c    |   27 +++++++++++++++++++++++++++
>  mm/truncate.c |   13 +++++++++++--
>  2 files changed, 38 insertions(+), 2 deletions(-)
...
> @@ -433,7 +434,15 @@ void truncate_inode_pages_final(struct address_space *mapping)
>  		spin_lock_irq(&mapping->tree_lock);
>  		spin_unlock_irq(&mapping->tree_lock);
>  
> -		truncate_inode_pages(mapping, 0);
> +		/*
> +		 * In the case of DAX we also need to unmap the inode
> +		 * since the pfn backing the mapping may be invalidated
> +		 * after this returns
> +		 */
> +		if (IS_DAX(inode))
> +			truncate_pagecache(inode, 0);
> +		else
> +			truncate_inode_pages(mapping, 0);
>  	}

Hum, I don't get this. truncate_inode_pages_final() gets called when inode
has no more users. So there are no mappings of the inode. So how could
truncate_pagecache() possibly make a difference?

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