Re: [PATCH 04/18] dax: Introduce IOMAP_DAX_COW to CoW edges during writes

[Jan Kara <jack@suse.cz>]

On Thu 30-05-19 08:14:45, Dave Chinner wrote:
> On Wed, May 29, 2019 at 03:46:29PM +0200, Jan Kara wrote:
> > On Wed 29-05-19 14:46:58, Dave Chinner wrote:
> > >  iomap_apply()
> > > 
> > >  	->iomap_begin()
> > > 		map old data extent that we copy from
> > > 
> > > 		allocate new data extent we copy to in data fork,
> > > 		immediately replacing old data extent
> > > 
> > > 		return transaction handle as private data
> 
> This holds the inode block map locked exclusively across the IO,
> so....

Does it? We do hold XFS_IOLOCK_EXCL during the whole dax write. But
xfs_file_iomap_begin() does release XFS_ILOCK_* on exit AFAICS. So I don't
see anything that would prevent page fault from mapping blocks into page
tables just after xfs_file_iomap_begin() returns.

> > > 	dax_iomap_actor()
> > > 		copies data from old extent to new extent
> > > 
> > > 	->iomap_end
> > > 		commits transaction now data has been copied, making
> > > 		the COW operation atomic with the data copy.
> > > 
> > > 
> > > This, in fact, should be how we do all DAX writes that require
> > > allocation, because then we get rid of the need to zero newly
> > > allocated or unwritten extents before we copy the data into it. i.e.
> > > we only need to write once to newly allocated storage rather than
> > > twice.
> > 
> > You need to be careful though. You need to synchronize with page faults so
> > that they cannot see and expose in page tables blocks you've allocated
> > before their contents is filled.
> 
> ... so the page fault will block trying to map the blocks because
> it can't get the xfs_inode->i_ilock until the allocation transaciton
> commits....
> 
> > This race was actually the strongest
> > motivation for pre-zeroing of blocks. OTOH copy_from_iter() in
> > dax_iomap_actor() needs to be able to fault pages to copy from (and these
> > pages may be from the same file you're writing to) so you cannot just block
> > faulting for the file through I_MMAP_LOCK.
> 
> Right, it doesn't take the I_MMAP_LOCK, but it would block further
> in. And, really, I'm not caring all this much about this corner
> case. i.e.  anyone using a "mmap()+write() zero copy" pattern on DAX
> within a file is unbeleivably naive - the data still gets copied by
> the CPU in the write() call. It's far simpler and more effcient to
> just mmap() both ranges of the file(s) and memcpy() in userspace....
> 
> FWIW, it's to avoid problems with stupid userspace stuff that nobody
> really should be doing that I want range locks for the XFS inode
> locks.  If userspace overlaps the ranges and deadlocks in that case,
> they they get to keep all the broken bits because, IMO, they are
> doing something monumentally stupid. I'd probably be making it
> return EDEADLOCK back out to userspace in the case rather than
> deadlocking but, fundamentally, I think it's broken behaviour that
> we should be rejecting with an error rather than adding complexity
> trying to handle it.

I agree with this. We must just prevent user from taking the kernel down
with maliciously created IOs...

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