linux-kernel - Re: [PATCH v5 2/2] buffer: record blockdev write errors in super

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Date:   Wed, 15 Apr 2020 16:06:42 +0200
From:   Jan Kara <jack@...e.cz>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        andres@...razel.de, willy@...radead.org, dhowells@...hat.com,
        hch@...radead.org, jack@...e.cz, akpm@...ux-foundation.org,
        david@...morbit.com
Subject: Re: [PATCH v5 2/2] buffer: record blockdev write errors in
 super_block that it backs

On Wed 15-04-20 08:13:00, Jeff Layton wrote:
> From: Jeff Layton <jlayton@...hat.com>
> 
> When syncing out a block device (a'la __sync_blockdev), any error
> encountered will only be recorded in the bd_inode's mapping. When the
> blockdev contains a filesystem however, we'd like to also record the
> error in the super_block that's stored there.
> 
> Make mark_buffer_write_io_error also record the error in the
> corresponding super_block when a writeback error occurs and the block
> device contains a mounted superblock.
> 
> Since superblocks are RCU freed, hold the rcu_read_lock to ensure
> that the superblock doesn't go away while we're marking it.
> 
> Signed-off-by: Jeff Layton <jlayton@...nel.org>
> ---
>  fs/buffer.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index f73276d746bb..2a4a5cc20418 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1154,12 +1154,19 @@ EXPORT_SYMBOL(mark_buffer_dirty);
>  
>  void mark_buffer_write_io_error(struct buffer_head *bh)
>  {
> +	struct super_block *sb;
> +
>  	set_buffer_write_io_error(bh);
>  	/* FIXME: do we need to set this in both places? */
>  	if (bh->b_page && bh->b_page->mapping)
>  		mapping_set_error(bh->b_page->mapping, -EIO);
>  	if (bh->b_assoc_map)
>  		mapping_set_error(bh->b_assoc_map, -EIO);
> +	rcu_read_lock();
> +	sb = bh->b_bdev->bd_super;

You still need READ_ONCE() here. Otherwise the dereference below can still
result in refetch and NULL ptr deref.

								Honza

> +	if (sb)
> +		errseq_set(&sb->s_wb_err, -EIO);
> +	rcu_read_unlock();
>  }
>  EXPORT_SYMBOL(mark_buffer_write_io_error);
>  
> -- 
> 2.25.2
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR