linux-kernel - Re: [PATCH 2/5] buffer: record blockdev write errors in super

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Date:   Wed, 06 Jun 2018 11:56:06 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     viro@...IV.linux.org.uk, dhowells@...hat.com,
        Jens Axboe <axboe@...com>
Cc:     willy@...radead.org, andres@...razel.de, cmaiolino@...hat.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Subject: Re: [PATCH 2/5] buffer: record blockdev write errors in super_block
 that backs them

On Mon, 2018-06-04 at 14:03 -0400, 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.
> 
> Signed-off-by: Jeff Layton <jlayton@...hat.com>
> ---
>  fs/buffer.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 249b83fafe48..dae2a857d5bc 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1117,6 +1117,8 @@ void mark_buffer_write_io_error(struct buffer_head *bh)
>  		mapping_set_error(bh->b_page->mapping, -EIO);
>  	if (bh->b_assoc_map)
>  		mapping_set_error(bh->b_assoc_map, -EIO);
> +	if (bh->b_bdev->bd_super)
> +		errseq_set(&bh->b_bdev->bd_super->s_wb_err, -EIO);
>  }
>  EXPORT_SYMBOL(mark_buffer_write_io_error);
>  

(cc'ing linux-block and Jens)

I'm wondering whether this patch might turn out to be racy. For
instance, could a call to __sync_blockdev race with an unmount in such
a way that bd_super goes NULL after we check it but before errseq_set
is called?

If so, what can we do to ensure that that doesn't happen? Any insight
here would be appreciated.

Thanks,
-- 
Jeff Layton <jlayton@...nel.org>