Re: [PATCH 3/8] block: remove the racy bd_inode->i_mapping->nrpages asserts

[Christoph Hellwig <hch@lst.de>]

On Thu, Jan 27, 2022 at 01:23:27PM +0100, Jan Kara wrote:
> On Thu 27-01-22 10:49:42, Christoph Hellwig wrote:
> > On Thu, Jan 27, 2022 at 10:47:37AM +0100, Jan Kara wrote:
> > > On Wed 26-01-22 16:50:35, Christoph Hellwig wrote:
> > > > Nothing prevents a file system or userspace opener of the block device
> > > > from redirtying the page right afte sync_blockdev returned.  Fortunately
> > > > data in the page cache during a block device change is mostly harmless
> > > > anyway.
> > > > 
> > > > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > > 
> > > My understanding was these warnings are there to tell userspace it is doing
> > > something wrong. Something like the warning we issue when DIO races with
> > > buffered IO... I'm not sure how useful they are but I don't see strong
> > > reason to remove them either...
> > 
> > Well, it is not just a warning, but also fails the command.  With some of
> > the reduced synchronization blktests loop/002 can hit them pretty reliably.
> 
> I see. I guess another place where using mapping->invalidate_lock would be
> good to avoid these races... So maybe something like attached patch?

So this looks sensible, but it does nest the inode lock and and the
invalidate lockinside lo_mutex.  I wonder if that is going to create
more problems down the road.