linux-ext4 - Re: [PATCH] ext4: Unmap metadata when zeroing blocks

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Message-ID: <20160929131457.wyhlp34n55ewzzxu@localhost.localdomain>
Date:   Thu, 29 Sep 2016 15:14:57 +0200
From:   Lukas Czerner <lczerner@...hat.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ted Tso <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: Unmap metadata when zeroing blocks

Hi Jan,

do we have a xfstest case for this particular problem ?

Thanks!
-Lukas


On Tue, Sep 27, 2016 at 06:15:18PM +0200, Jan Kara wrote:
> When zeroing blocks for DAX allocations, we also have to unmap aliases
> in the block device mappings. Otherwise writeback can overwrite zeros
> with stale data from block device page cache.
> 
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>  fs/ext4/inode.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> Ted, can you merge this patch please? It's a data corruption issue for DAX.
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index c6ea25a190f8..87150122d361 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -647,11 +647,19 @@ found:
>  		/*
>  		 * We have to zeroout blocks before inserting them into extent
>  		 * status tree. Otherwise someone could look them up there and
> -		 * use them before they are really zeroed.
> +		 * use them before they are really zeroed. We also have to
> +		 * unmap metadata before zeroing as otherwise writeback can
> +		 * overwrite zeros with stale data from block device.
>  		 */
>  		if (flags & EXT4_GET_BLOCKS_ZERO &&
>  		    map->m_flags & EXT4_MAP_MAPPED &&
>  		    map->m_flags & EXT4_MAP_NEW) {
> +			ext4_lblk_t i;
> +
> +			for (i = 0; i < map->m_len; i++) {
> +				unmap_underlying_metadata(inode->i_sb->s_bdev,
> +							  map->m_pblk + i);
> +			}
>  			ret = ext4_issue_zeroout(inode, map->m_lblk,
>  						 map->m_pblk, map->m_len);
>  			if (ret) {
> -- 
> 2.6.6
> 
> --
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