On Tue, 2018-06-19 at 06:40 -0400, Jeff Layton wrote: > 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@xxxxxxxxxx> > > > > > > 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@xxxxxxxxxx> > > > --- > > > 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? > (cc'ing Ted since he added blkdev_releasepage in 2009) Corollary question: What makes it safe to dereference bd_super in blkdev_releasepage? bd_super can go NULL in kill_sb and eventually the super_block will be freed. Is there a ToC/ToU race in that function? -- Jeff Layton <jlayton@xxxxxxxxxx>
