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

[Dan Williams]

On Tue, Feb 2, 2016 at 9:34 AM, Ross Zwisler
<ross.zwisler@xxxxxxxxxxxxxxx> wrote:
> 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.

Ugh, and we can't trigger it from where a filesystem normally syncs a
block device, becauDid you tryse we lose track of the inode radix
information at that level.

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()