linux-kernel - [PATCH v6 7/9] dax: Check for data cache aliasing at runtime

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Message-Id: <20240215144633.96437-8-mathieu.desnoyers@efficios.com>
Date: Thu, 15 Feb 2024 09:46:31 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Dan Williams <dan.j.williams@...el.com>,
	Arnd Bergmann <arnd@...db.de>,
	Dave Chinner <david@...morbit.com>
Cc: linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Matthew Wilcox <willy@...radead.org>,
	Russell King <linux@...linux.org.uk>,
	linux-arch@...r.kernel.org,
	linux-cxl@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org,
	linux-xfs@...r.kernel.org,
	dm-devel@...ts.linux.dev,
	nvdimm@...ts.linux.dev,
	linux-s390@...r.kernel.org
Subject: [PATCH v6 7/9] dax: Check for data cache aliasing at runtime

Replace the following fs/Kconfig:FS_DAX dependency:

  depends on !(ARM || MIPS || SPARC)

By a runtime check within alloc_dax(). This runtime check returns
ERR_PTR(-EOPNOTSUPP) if the @ops parameter is non-NULL (which means
the kernel is using an aliased mapping) on an architecture which
has data cache aliasing.

Change the return value from NULL to PTR_ERR(-EOPNOTSUPP) for
CONFIG_DAX=n for consistency.

This is done in preparation for using cpu_dcache_is_aliasing() in a
following change which will properly support architectures which detect
data cache aliasing at runtime.

Fixes: d92576f1167c ("dax: does not work correctly with virtual aliasing caches")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Reviewed-by: Dan Williams <dan.j.williams@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Dan Williams <dan.j.williams@...el.com>
Cc: Vishal Verma <vishal.l.verma@...el.com>
Cc: Dave Jiang <dave.jiang@...el.com>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Russell King <linux@...linux.org.uk>
Cc: linux-arch@...r.kernel.org
Cc: linux-cxl@...r.kernel.org
Cc: linux-fsdevel@...r.kernel.org
Cc: linux-mm@...ck.org
Cc: linux-xfs@...r.kernel.org
Cc: dm-devel@...ts.linux.dev
Cc: nvdimm@...ts.linux.dev
---
 drivers/dax/super.c | 10 ++++++++++
 fs/Kconfig          |  1 -
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index 205b888d45bf..ce5bffa86bba 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -450,6 +450,16 @@ struct dax_device *alloc_dax(void *private, const struct dax_operations *ops)
 	dev_t devt;
 	int minor;
 
+	/*
+	 * Unavailable on architectures with virtually aliased data caches,
+	 * except for device-dax (NULL operations pointer), which does
+	 * not use aliased mappings from the kernel.
+	 */
+	if (ops && (IS_ENABLED(CONFIG_ARM) ||
+	    IS_ENABLED(CONFIG_MIPS) ||
+	    IS_ENABLED(CONFIG_SPARC)))
+		return ERR_PTR(-EOPNOTSUPP);
+
 	if (WARN_ON_ONCE(ops && !ops->zero_page_range))
 		return ERR_PTR(-EINVAL);
 
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
-- 
2.39.2