Re: [PATCH] dax: allow DAX to look up an inode's block device

[Dan Williams]

On Tue, Feb 2, 2016 at 3:36 PM, Jared Hulbert <jaredeh@xxxxxxxxx> wrote:
> On Tue, Feb 2, 2016 at 3:19 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>>
>> On Tue, Feb 02, 2016 at 04:11:42PM -0700, Ross Zwisler wrote:
>>
>> > However, for raw block devices and for XFS with a real-time device, the
>> > value in inode->i_sb->s_bdev is not correct.  With the code as it is
>> > currently written, an fsync or msync to a DAX enabled raw block device will
>> > cause a NULL pointer dereference kernel BUG.  For this to work correctly we
>> > need to ask the block device or filesystem what struct block_device is
>> > appropriate for our inode.
>> >
>> > To that end, add a get_bdev(struct inode *) entry point to struct
>> > super_operations.  If this function pointer is non-NULL, this notifies DAX
>> > that it needs to use it to look up the correct block_device.  If
>> > i_sb->get_bdev() is NULL DAX will default to inode->i_sb->s_bdev.
>>
>> Umm...  It assumes that bdev will stay pinned for as long as inode is
>> referenced, presumably?  If so, that needs to be documented (and verified
>> for existing fs instances).  In principle, multi-disk fs might want to
>> support things like "silently move the inodes backed by that disk to other
>> ones"...
>
> Dan, This is exactly the kind of thing I'm taking about WRT the
> weirder device models and directly calling bdev_direct_access().
> Filesystems don't have the monogamous relationship with a device that
> is implicitly assumed in DAX, you have to ask the filesystem what the
> relationship is and is migrating to, and allow the filesystem to
> update DAX when the relationship is changing.

That's precisely what ->get_bdev() does.  When the answer
inode->i_sb->s_bdev lookup is invalid, use ->get_bdev().

> As we start to see many
> DIMM's and 10s TiB pmem systems this is going be an even bigger deal
> as load balancing, wear leveling, and fault tolerance concerned are
> inevitably driven by the filesystem.

No, there are no plans on the horizon for an fs to manage these media
specific concerns for persistent memory.