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

[Ross Zwisler]

On Tue, Feb 02, 2016 at 09:10:24AM -0800, Dan Williams wrote:
> On Tue, Feb 2, 2016 at 8:46 AM, Jan Kara <jack@xxxxxxx> wrote:
> > On Tue 02-02-16 08:33:56, Dan Williams wrote:
> >> On Tue, Feb 2, 2016 at 3:17 AM, Jan Kara <jack@xxxxxxx> wrote:
> >> [..]
> >> > I see, thanks for explanation. So I'm OK with changing what is stored in
> >> > the radix tree to accommodate this use case but my reservation that we IHMO
> >> > have other more pressing things to fix remains...
> >>
> >> We don't need pfns in the radix to support XFS RT configurations.
> >> Just call get_blocks() again and use the sector, or am I missing
> >> something?
> >
> > You are correct. But if you decide to pay the cost of additional
> > get_block() call, you only need the dirty tag in the radix tree and nothing
> > else. So my understanding was that the whole point of games with radix tree
> > is avoiding this extra get_block() calls for fsync().
> >
> 
> DAX-fsync() is already a potentially expensive operation to cover data
> durability guarantees for DAX-unaware applications.  A DAX-aware
> application is going to skip fsync, and the get_blocks() cost, to do
> cache management itself.
> 
> Willy pointed out some other potential benefits, assuming a suitable
> replacement for the protections afforded by the block-device
> percpu_ref counter can be found.  However, optimizing for the
> DAX-unaware-application case seems the wrong motivation to me.

Oh, no, the primary issue with calling get_block() in the fsync path isn't
performance.  It's that we don't have any idea what get_block() function to
call.

The fault handler calls all come from the filesystem directly, so they can
easily give us an appropriate get_block() function pointer.  But the
dax_writeback_mapping_range() calls come from the generic code in
mm/filemap.c, and don't know what get_block() to pass in.

During one iteration I had the calls to dax_writeback_mapping_range()
happening in the filesystem fsync code so that it could pass in get_block(),
but Dave Chinner pointed out that this misses other paths in the filesystem
that need to have things flushed via a call to filemap_write_and_wait_range().

In yet another iteration of this series I tried adding get_block() to struct
inode_operations so that I could access it from what is now
dax_writeback_mapping_range(), but this was shot down as well.