Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

[Ross Zwisler]

On Thu, Feb 04, 2016 at 09:29:57PM +0100, Jan Kara wrote:
> On Thu 04-02-16 12:56:19, Ross Zwisler wrote:
> > On Wed, Feb 03, 2016 at 11:46:11AM +0100, Jan Kara wrote:
<>
> > > Let's clear this up a bit: The problem with using ->fsync() method is that
> > > it doesn't get called for sync(2). We could use ->sync_fs() to flush caches
> > > in case of sync(2) (that's what's happening for normal storage) but the
> > > problem with PMEM is that "flush all cached data" operation effectively
> > > means iterate through all modified pages and we didn't want to implement
> > > this for DAX fsync code.
> > > 
> > > So we have decided to do cache flushing for DAX at a different point - mark
> > > inodes which may have writes cached as dirty and use writeback code for the
> > > cache flushing. But looking at it now, we have actually chosen a wrong
> > > place to do the flushing in the writeback path - note that sync(2) writes
> > > data via __writeback_single_inode() -> do_writepages() and thus doesn't
> > > even get to filemap_write_and_wait().
> > > 
> > > So revisiting the decision I see two options:
> > > 
> > > 1) Move the DAX flushing code from filemap_write_and_wait() into
> > > ->writepages() fs callback. There the filesystem can provide all the
> > > information it needs including bdev, get_block callback, or whatever.
> > > 
> > > 2) Back out even further and implement own tracking and iteration of inodes
> > > to write.
> > > 
> > > So far I still think 2) is not worth the complexity (although it would
> > > bring DAX code closer to how things behave with standard storage) so I
> > > would go for 1).
> > 
> > Jan, just to clarify, are you proposing this change for v4.5 in the remaining
> > RCs as an alternative to the get_bdev() patch?
> > 
> > https://lkml.org/lkml/2016/2/2/941
> 
> Yes, because I don't think anything like ->get_bdev() is needed at all.
> Look: dax_do_io(), __dax_fault(), __dax_pmd_fault(), dax_zero_page_range()
> don't really need bdev - we have agreed that get_block() fills that in just
> fine.
> 
> dax_clear_blocks() has IMO just the wrong signature - it should take bdev
> and not inode as an argument. Because combination inode + bdev sector
> doesn't really make much sense.
> 
> dax_writeback_mapping_range() is the only remaining offender and it can
> easily take bdev as an argument when called from ->writepages().
> 
> > Or can we move forward with get_bdev(), and try and figure out this new way of
> > calling fsync/msync for v4.6?  My main concern here is that changing how the
> > DAX sync code gets called will affect all three filesystems as well as MM, and
> > that it might be too much for RC inclusion...
> 
> I think changes aren't very intrusive so we can feed them in during RC
> phase and frankly, you have to move to using ->writepages() anyway to make
> sync(2) work reliably.

Okay, sounds good.  I'll send it out once I've got it working & tested.