[PATCH] ext4: Unmap metadata when zeroing blocks

[PATCH] ext4: Unmap metadata when zeroing blocks

[Jan Kara]

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@suse.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

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

[Lukas Czerner]

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@suse.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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Re: [PATCH] ext4: Unmap metadata when zeroing blocks

[Jan Kara]

Hi,

On Thu 29-09-16 15:14:57, Lukas Czerner wrote:
> do we have a xfstest case for this particular problem ?

Not specifically, but some of the existing xfstests did trigger this
problem for me which is how I found it out...

								Honza

> 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@suse.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
> > 
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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

[Theodore Ts'o]

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@suse.cz>

Thanks, applied.

					- Ted