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

[Dan Williams]

On Tue, Feb 2, 2016 at 10:46 AM, Matthew Wilcox <willy@xxxxxxxxxxxxxxx> 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).

True, but in this case we're just trying to resolve the bdev for a
inode / sector combination to already allocated storage.  So
get_block() is a misnomer, this is just get_bdev() to resolve a
super_block-inode+sector tuple to a bdev for cases when s_bdev is the
wrong answer.