Re: [PATCH 4/8] mm, dax: truncate dax mappings at bdev or fs shutdown

[Jan Kara <jack@suse.cz>]

On Thu 19-11-15 08:55:48, Dan Williams wrote:
> On Thu, Nov 19, 2015 at 4:55 AM, Jan Kara <jack@suse.cz> wrote:
> > Also are you sure that your unmapping cannot race with other process
> > mapping the pfns again?
> 
> You're right, there is indeed a gap between the unmap and when
> get_blocks() will start returning errors in the fault path.  I think I
> need to defer the unmapping until after blk_cleanup_queue() where we
> know that no new I/o to the device is possible.

Yeah, you need to squeeze it somewhere after the point where get_blocks()
start returning errors and before the point where pfn can go away.

> > BTW what happens when you access a DAX pfn that got removed?
> 
> SIGBUS.  I don't see a way to be any kinder to the application.  After
> the ->remove() method for the block_device is complete we can't be
> sure that the pfn is valid or even present in the system (think brd,
> or VM hot provisioning).

I see. So if we kept the PFN mapped, it could result e.g. in memory
corruption (at least in case of brd). So we really need this to be 100%
reliable. That's what I was interested in.

									Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR