linux-kernel - Re: [PATCH] dm: writecache: add DAX dependency

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Date:   Tue, 29 May 2018 09:17:12 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Mikulas Patocka <mpatocka@...hat.com>,
        Shaohua Li <shli@...nel.org>, Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-fsdevel@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>,
        Heinz Mauelshagen <heinzm@...hat.com>,
        linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dm: writecache: add DAX dependency

On Mon, May 28, 2018 at 05:38:10PM +0200, Arnd Bergmann wrote:
> The new dm-writecache driver inconditionally uses the dax
			       unconditionally

> subsystem, leading to link errors in some configurations:
> 
> drivers/md/dm-writecache.o: In function `writecache_ctr':
> dm-writecache.c:(.text+0x1fdc): undefined reference to `dax_read_lock'
> dm-writecache.c:(.text+0x2004): undefined reference to `dax_direct_access'
> dm-writecache.c:(.text+0x21cc): undefined reference to `dax_read_unlock'
> 
> It seems wrong to require DAX in order to build the writecache
> driver, but that at least avoids randconfig build errors.
> 
> Fixes: bb15b431d650 ("dm: add writecache target")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/md/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
> index 852c7ebe2902..f8ecf2da1edf 100644
> --- a/drivers/md/Kconfig
> +++ b/drivers/md/Kconfig
> @@ -338,6 +338,7 @@ config DM_CACHE_SMQ
>  config DM_WRITECACHE
>  	tristate "Writecache target"
>  	depends on BLK_DEV_DM
> +	depends on DAX
>  	---help---
>  	   The writecache target caches writes on persistent memory or SSD.
>  	   It is intended for databases or other programs that need extremely

I think the right way to handle this is to instead wrap all the DAX code in
dm-writecache in "#if IS_ENABLED(CONFIG_DAX_DRIVER)" blocks and provide stubs
for the non-DAX case.  This prevents you from having to pull in all the
generic DAX code unnecessarily.

In looking at the file I think this is just the persistent_memory_claim()
function.

I'll send out a patch once I've tested a bit.

- Ross