[RFC PATCH v4 06/12] dax: Check for data cache aliasing at runtime

[Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>]

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@xxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Vishal Verma <vishal.l.verma@xxxxxxxxx>
Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: linux-arch@xxxxxxxxxxxxxxx
Cc: linux-cxl@xxxxxxxxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: linux-xfs@xxxxxxxxxxxxxxx
Cc: dm-devel@xxxxxxxxxxxxxxx
Cc: nvdimm@xxxxxxxxxxxxxxx
---
 drivers/dax/super.c | 15 +++++++++++++++
 fs/Kconfig          |  1 -
 include/linux/dax.h |  6 +-----
 3 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index 0da9232ea175..ce5bffa86bba 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -319,6 +319,11 @@ EXPORT_SYMBOL_GPL(dax_alive);
  * that any fault handlers or operations that might have seen
  * dax_alive(), have completed.  Any operations that start after
  * synchronize_srcu() has run will abort upon seeing !dax_alive().
+ *
+ * Note, because alloc_dax() returns an ERR_PTR() on error, callers
+ * typically store its result into a local variable in order to check
+ * the result. Therefore, care must be taken to populate the struct
+ * device dax_dev field make sure the dax_dev is not leaked.
  */
 void kill_dax(struct dax_device *dax_dev)
 {
@@ -445,6 +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
diff --git a/include/linux/dax.h b/include/linux/dax.h
index b463502b16e1..df2d52b8a245 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -86,11 +86,7 @@ static inline void *dax_holder(struct dax_device *dax_dev)
 static inline struct dax_device *alloc_dax(void *private,
 		const struct dax_operations *ops)
 {
-	/*
-	 * Callers should check IS_ENABLED(CONFIG_DAX) to know if this
-	 * NULL is an error or expected.
-	 */
-	return NULL;
+	return ERR_PTR(-EOPNOTSUPP);
 }
 static inline void put_dax(struct dax_device *dax_dev)
 {
-- 
2.39.2