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

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Date:	Tue, 2 Feb 2016 10:59:41 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Matthew Wilcox <willy@...ux.intel.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-nvdimm <linux-nvdimm@...1.01.org>
Subject: Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

On Tue, Feb 2, 2016 at 10:46 AM, Matthew Wilcox <willy@...ux.intel.com> 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.