linux-kernel - Re: [PATCH v2 2/5] dax: clear TOWRITE flag after flush is complete

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Date:	Fri, 22 Jan 2016 15:55:59 +0100
From:	Jan Kara <jack@...e.cz>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-fsdevel@...r.kernel.org, linux-nvdimm@...ts.01.org
Subject: Re: [PATCH v2 2/5] dax: clear TOWRITE flag after flush is complete

On Thu 21-01-16 10:46:01, Ross Zwisler wrote:
> Previously in dax_writeback_one() we cleared the PAGECACHE_TAG_TOWRITE flag
> before we had actually flushed the tagged radix tree entry to media.  This
> is incorrect because of the following race:
> 
> Thread 1				Thread 2
> --------				--------
> dax_writeback_mapping_range()
> tag entry with PAGECACHE_TAG_TOWRITE
> 					dax_writeback_mapping_range()
> 					tag entry with PAGECACHE_TAG_TOWRITE
> 					dax_writeback_one()
> 					radix_tree_tag_clear(TOWRITE)
> TOWRITE flag is no longer set,
>   find_get_entries_tag() finds no
>   entries, return
> 					flush entry to media
> 
> In this case thread 1 returns before the data for the dirty entry is
> actually durable on media.
> 
> Fix this by only clearing the PAGECACHE_TAG_TOWRITE flag after all flushing
> is complete.
> 
> Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> Reported-by: Jan Kara <jack@...e.cz>

Looks good. You can add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza
> ---
>  fs/dax.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index cee9e1b..d589113 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -407,8 +407,6 @@ static int dax_writeback_one(struct block_device *bdev,
>  	if (!radix_tree_tag_get(page_tree, index, PAGECACHE_TAG_TOWRITE))
>  		goto unlock;
>  
> -	radix_tree_tag_clear(page_tree, index, PAGECACHE_TAG_TOWRITE);
> -
>  	if (WARN_ON_ONCE(type != RADIX_DAX_PTE && type != RADIX_DAX_PMD)) {
>  		ret = -EIO;
>  		goto unlock;
> @@ -432,6 +430,10 @@ static int dax_writeback_one(struct block_device *bdev,
>  	}
>  
>  	wb_cache_pmem(dax.addr, dax.size);
> +
> +	spin_lock_irq(&mapping->tree_lock);
> +	radix_tree_tag_clear(page_tree, index, PAGECACHE_TAG_TOWRITE);
> +	spin_unlock_irq(&mapping->tree_lock);
>   unmap:
>  	dax_unmap_atomic(bdev, &dax);
>  	return ret;
> -- 
> 2.5.0
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR