linux-kernel - Re: [PATCH] dax: try to avoid unused function warnings

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Message-ID: <CAPcyv4iHKqYcq2OZZynUzMe8aF4JiKCUZa2JnERGC9MC7aY8Lg@mail.gmail.com>
Date:   Mon, 28 Nov 2016 14:13:29 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>, "Theodore Ts'o" <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...ux.intel.com>,
        Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Chinner <dchinner@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dax: try to avoid unused function warnings

On Mon, Nov 28, 2016 at 1:24 PM, Ross Zwisler
<ross.zwisler@...ux.intel.com> wrote:
> On Mon, Nov 28, 2016 at 10:12:17PM +0100, Arnd Bergmann wrote:
>> Without the get_block based I/O, we get warnings when CONFIG_FS_IOMAP
>> is disabled:
>>
>> fs/dax.c:736:12: error: ‘dax_insert_mapping’ defined but not used [-Werror=unused-function]
>> fs/dax.c:512:12: error: ‘copy_user_dax’ defined but not used [-Werror=unused-function]
>> fs/dax.c:490:12: error: ‘dax_load_hole’ defined but not used [-Werror=unused-function]
>> fs/dax.c:294:14: error: ‘grab_mapping_entry’ defined but not used [-Werror=unused-function]
>>
>> This patch blindly marks those as __maybe_unused, which avoids the warnings.
>> However, I suspect that there is actually more code in this file that should
>> not be provided without CONFIG_FS_IOMAP even though we don't get a warning
>> for it, and that we actually want a different rework, so please treat this
>> as a bug report. I have applied the patch locally in my randconfig build
>> setup to avoid seeing the warnings.
>>
>> Fixes: 5ac65736f740 ("dax: rip out get_block based IO support")
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>
> Thanks for the report.  I think the right way to deal with this is to just
> select FS_IOMAP when we pull in the DAX code.  I sent out a patch last week
> that does this:
>
> https://lkml.org/lkml/2016/11/23/591

It seems awkward for both filesystems and the FS_DAX core to be
selecting FS_IOMAP.  In the end FS_DAX and FS_IOMAP are both libraries
of functionality that a filesystem can optionally use.  I think the
longer term FS_DAX stops being an independent user visible setting and
is instead selected by filesystems that want DAX.