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Date: Wed, 31 Jan 2024 13:38:09 +1100
From: Dave Chinner <david@fromorbit.com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Dave Jiang <dave.jiang@intel.com>, linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>, linux-mm@kvack.org,
	linux-arch@vger.kernel.org, Matthew Wilcox <willy@infradead.org>,
	Arnd Bergmann <arnd@arndb.de>, Russell King <linux@armlinux.org.uk>,
	nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [RFC PATCH v2 1/8] dax: Introduce dax_is_supported()
Message-ID: <ZbmykQpcllC/LY6J@dread.disaster.area>
References: <20240130165255.212591-1-mathieu.desnoyers@efficios.com>
 <20240130165255.212591-2-mathieu.desnoyers@efficios.com>
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In-Reply-To: <20240130165255.212591-2-mathieu.desnoyers@efficios.com>

On Tue, Jan 30, 2024 at 11:52:48AM -0500, Mathieu Desnoyers wrote:
> Introduce a new dax_is_supported() static inline to check whether the
> architecture supports DAX.
> 
> This replaces the following fs/Kconfig:FS_DAX dependency:
> 
>   depends on !(ARM || MIPS || SPARC)
> 
> This is done in preparation for its use by each filesystem supporting
> the dax mount option to validate whether dax is indeed supported.
> 
> This is done in preparation for using dcache_is_aliasing() in a
> following change which will properly support architectures which detect
> dcache aliasing at runtime.
> 
> Fixes: d92576f1167c ("dax: does not work correctly with virtual aliasing caches")
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: linux-mm@kvack.org
> Cc: linux-arch@vger.kernel.org
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Vishal Verma <vishal.l.verma@intel.com>
> Cc: Dave Jiang <dave.jiang@intel.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Russell King <linux@armlinux.org.uk>
> Cc: nvdimm@lists.linux.dev
> Cc: linux-cxl@vger.kernel.org
> Cc: linux-fsdevel@vger.kernel.org
> ---
>  fs/Kconfig          |  1 -
>  include/linux/dax.h | 10 ++++++++++
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/Kconfig b/fs/Kconfig
> index 42837617a55b..e5efdb3b276b 100644
> --- a/fs/Kconfig
> +++ b/fs/Kconfig
> @@ -56,7 +56,6 @@ endif # BLOCK
>  config FS_DAX
>  	bool "File system based Direct Access (DAX) support"
>  	depends on MMU
> -	depends on !(ARM || MIPS || SPARC)
>  	depends on ZONE_DEVICE || FS_DAX_LIMITED
>  	select FS_IOMAP
>  	select DAX
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index b463502b16e1..cfc8cd4a3eae 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -78,6 +78,12 @@ static inline bool daxdev_mapping_supported(struct vm_area_struct *vma,
>  		return false;
>  	return dax_synchronous(dax_dev);
>  }
> +static inline bool dax_is_supported(void)
> +{
> +	return !IS_ENABLED(CONFIG_ARM) &&
> +	       !IS_ENABLED(CONFIG_MIPS) &&
> +	       !IS_ENABLED(CONFIG_SPARC);
> +}

Uh, ok. Now I see what dax_is_supported() does.

I think this should be folded into fs_dax_get_by_bdev(), which
currently returns NULL if CONFIG_FS_DAX=n and so should be cahnged
to return NULL if any of these platform configs is enabled.

Then I don't think you need to change a single line of filesystem
code - they'll all just do what they do now if the block device
doesn't support DAX....

-Dave.
-- 
Dave Chinner
david@fromorbit.com