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Date:   Wed, 30 May 2018 08:21:24 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     Arnd Bergmann <arnd@arndb.de>
cc:     Shaohua Li <shli@kernel.org>, Alasdair Kergon <agk@redhat.com>,
        Mike Snitzer <snitzer@redhat.com>, dm-devel@redhat.com,
        Matthew Wilcox <mawilcox@microsoft.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        linux-fsdevel@vger.kernel.org,
        Dan Williams <dan.j.williams@intel.com>,
        Heinz Mauelshagen <heinzm@redhat.com>,
        linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] dm: writecache: add DAX dependency
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On Mon, 28 May 2018, Arnd Bergmann wrote:

> The new dm-writecache driver inconditionally uses the dax
> 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@arndb.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
> -- 
> 2.9.0

dm-writecache may be used without DAX in SSD-only mode.

So, I'd fix this by changing the code in dm-writecache.c to

#if !defined(CONFIG_ARCH_HAS_PMEM_API) || !defined(CONFIG_DAX)
#define DM_WRITECACHE_ONLY_SSD
#endif

Mikulas