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

[Jan Kara]

On Tue 02-02-16 13:46:43, Matthew Wilcox wrote:
> On Tue, Feb 02, 2016 at 09:46:21AM -0800, Dan Williams wrote:
> > What a about a super_operation?  That seems the right level, given
> > we're currently doing:
> > 
> > inode->i_sb->s_bdev
> > 
> > ...it does not seem terrible to instead do:
> > 
> > inode->i_sb->s_op->get_block()
> 
> The point is that filesystems have lots of different get_block operations,
> and the right one to use depends not just on the inode, but also upon
> what VFS function is being called, and in some filesystems the phase
> of the moon, or the file open flags (so even inode->i_ops->get_block is
> wrong; file->f_ops->get_block would be better, but of course we've lost
> that by the point we're doing writeback).

See what I wrote to Ross. I think this particular issue needs to be solved
by moving the flushing to ->writepages() callback.

> I now realise that basing DAX around get_block & buffer_heads was a mistake.
> I think the Right Solution (not for 4.5) is to ask filesystems to populate
> the radix tree.  A flow somewhat like this:
> 
> 1. VFS or VM calls filesystem (eg ->fault())
> 2. Filesystem calls DAX (eg dax_fault())
> 3. DAX looks in radix tree, finds no information.
> 4. DAX calls (NEW!) mapping->a_ops->populate_pfns
> 5. Filesystem looks up its internal data structure (eg extent tree) and
>    calls dax_create_pfns() (see giant patch from yesterday, only instead of
>    passing a get_block_t, the filesystem has already filled in a bh which
>    describes the entire extent that this access happens to land in).
> 6. DAX continues to take care of calling bdev_direct_access() from
>    dax_create_pfns().
 
So I don't think that ->populate_pfns() is the right interface because it
doesn't really tell the filesystem what you want to do. It is essentially
like get_blocks() callback only you additionaly ask the fs to fill in the
mapping information into the radix tree. So it has the same problems as
get_blocks() callback in inode_operations (or superblock_operations,
aops, or anywhere else). History has taught us (there was get_blocks()
callback in inode operations in ancient times ;) that fs really needs to
know wider context to decide how exactly to fulfill the request.

I don't see anything obviously wrong with using radix tree as a primary
source of mapping information for DAX (after all we do that for page cache
as well where the mapping information is attached to pages in the radix
tree). But this seems independent of the get_blocks() vs something else
discussion.

And if your problem is with vaguely defined meaning of buffer_head flags
returned from get_blocks() callback, using the iomap interface (which XFS
currently uses for pNFS) would solve that.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR