linux-fsdevel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Jeff Layton <jlayton@kernel.org>
To: viro@ZenIV.linux.org.uk, dhowells@redhat.com,
	Jens Axboe <axboe@fb.com>, David Howells <dhowells@redhat.com>
Cc: willy@infradead.org, andres@anarazel.de, cmaiolino@redhat.com,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>
Subject: Re: [PATCH 2/5] buffer: record blockdev write errors in super_block that backs them
Date: Tue, 19 Jun 2018 06:40:23 -0400	[thread overview]
Message-ID: <0ddda59286e0be135cf133dc653da54f66c264a7.camel@kernel.org> (raw)
In-Reply-To: <81a365a631279f8b0ad0ed71b222c19817045704.camel@kernel.org>

On Wed, 2018-06-06 at 11:56 -0400, Jeff Layton wrote:
> On Mon, 2018-06-04 at 14:03 -0400, Jeff Layton wrote:
> > From: Jeff Layton <jlayton@redhat.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@redhat.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,

Jens, ping? I never got a response on the above.

After looking over it some more, I suspect that this may be racy with
some filesystems. Some of them seem to just flush out data to the
bd_inode on unmount, and trust the system to take care of the rest.

One possible fix there might be to turn bd_super into an RCU managed
pointer. We already free super_blocks under RCU, so we could do
something there like:

rcu_read_lock();
sb = rcu_dereference(bh->b_bdev->bd_super);
if (sb)
	errseq_set(&sb->s_wb_err, -EIO);
rcu_read_unlock();

There aren't that many accessors of bd_super, so that seems like it'd be
fairly simple to do.

Still, I'd like someone to sanity check me here. Is there something that
would prevent the above race that I'm not seeing?

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

  reply	other threads:[~2018-06-19 10:40 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-04 18:02 [PATCH 0/5] vfs: track per-sb writeback errors and report them via fsinfo() Jeff Layton
2018-06-04 18:03 ` [PATCH 1/5] vfs: track per-sb writeback errors Jeff Layton
2018-06-04 18:03 ` [PATCH 2/5] buffer: record blockdev write errors in super_block that backs them Jeff Layton
2018-06-06 15:56   ` Jeff Layton
2018-06-19 10:40     ` Jeff Layton [this message]
2018-06-19 13:03       ` Jeff Layton
2018-06-04 18:03 ` [PATCH 3/5] errseq: add a new errseq_scrape function Jeff Layton
2018-06-04 18:03 ` [PATCH 4/5] vfs: allow fsinfo to fetch the current state of s_wb_err Jeff Layton
2018-06-04 18:03 ` [PATCH 5/5] samples: extend test-fsinfo to access error_state Jeff Layton

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=0ddda59286e0be135cf133dc653da54f66c264a7.camel@kernel.org \
    --to=jlayton@kernel.org \
    --cc=andres@anarazel.de \
    --cc=axboe@fb.com \
    --cc=cmaiolino@redhat.com \
    --cc=dhowells@redhat.com \
    --cc=linux-block@vger.kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=viro@ZenIV.linux.org.uk \
    --cc=willy@infradead.org \
    --subject='Re: [PATCH 2/5] buffer: record blockdev write errors in super_block that backs them' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).