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Date: Tue, 2 Feb 2016 10:59:41 -0800
Message-ID: <CAPcyv4jf=4ea-C-Agh8FjDRJ1xkEpi0GTXFccdmwUADXfnAfrg@mail.gmail.com>
Subject: Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences
From: Dan Williams <dan.j.williams@intel.com>
To: Matthew Wilcox <willy@linux.intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>, Jan Kara <jack@suse.cz>,
        Dave Chinner <david@fromorbit.com>,
        Christoph Hellwig <hch@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-nvdimm <linux-nvdimm@ml01.01.org>
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On Tue, Feb 2, 2016 at 10:46 AM, Matthew Wilcox <willy@linux.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.