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 <j...@suse.com>
SUSE Labs, CR
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