On Tue, Feb 02, 2016 at 03:39:15PM -0800, Dan Williams 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"... > > I assume btrfs is the only fs we have that might reassign the bdev for > a given inode on the fly? Hopefully we don't need anything stronger > than rcu_read_lock() to pin the result as valid. > > At least in this case the initial user is dax-fsync where the > ->get_bdev() answer should be static for the life of the inode, and > btrfs does not currently interface with dax. But yes, we need to get > the expected semantics clear. Let's be clear though. ->get_bdev is a temporary hack. The need for it goes away when DAX doesn't rely on being on a block_device any more. I don't expect it to live longer than six months. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
