Re: [-mm PATCH v3 04/25] dax: fix lifetime of in-kernel dax mappings with dax_map_atomic()

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Fri, Dec 11, 2015 at 10:11:53AM -0800, Dan Williams wrote:
> The DAX implementation needs to protect new calls to ->direct_access()
> and usage of its return value against the driver for the underlying
> block device being disabled.  Use blk_queue_enter()/blk_queue_exit() to
> hold off blk_cleanup_queue() from proceeding, or otherwise fail new
> mapping requests if the request_queue is being torn down.
> 
> This also introduces blk_dax_ctl to simplify the interface from fs/dax.c
> through dax_map_atomic() to bdev_direct_access().
> 
> Cc: Jan Kara <jack@suse.com>
> Cc: Jens Axboe <axboe@fb.com>
> Cc: Dave Chinner <david@fromorbit.com>
> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
> Cc: Matthew Wilcox <willy@linux.intel.com>
> [willy: fix read() of a hole]
> Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
<>
> @@ -308,20 +351,18 @@ static int dax_insert_mapping(struct inode *inode, struct buffer_head *bh,
>  		goto out;
>  	}
>  
> -	error = bdev_direct_access(bh->b_bdev, sector, &addr, &pfn, bh->b_size);
> -	if (error < 0)
> -		goto out;
> -	if (error < PAGE_SIZE) {
> -		error = -EIO;
> +	if (dax_map_atomic(bdev, &dax) < 0) {
> +		error = PTR_ERR(dax.addr);
>  		goto out;
>  	}
>  
>  	if (buffer_unwritten(bh) || buffer_new(bh)) {
> -		clear_pmem(addr, PAGE_SIZE);
> +		clear_pmem(dax.addr, PAGE_SIZE);
>  		wmb_pmem();
>  	}
> +	dax_unmap_atomic(bdev, &dax);
>  
> -	error = vm_insert_mixed(vma, vaddr, pfn);
> +	error = vm_insert_mixed(vma, vaddr, dax.pfn);

Since we're still using the contents of the struct blk_dax_ctl as an argument
to vm_insert_mixed(), shouldn't dax_unmap_atomic() be after this call?

Unless there is some reason to protect dax.addr with a blk queue reference,
but not dax.pfn?

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