diff mbox series

[v5,1/8] dax: alloc_dax() return ERR_PTR(-EOPNOTSUPP) for CONFIG_DAX=n

Message ID 20240212163101.19614-2-mathieu.desnoyers@efficios.com
State Superseded
Headers show
Series Introduce cpu_dcache_is_aliasing() to fix DAX regression | expand

Commit Message

Mathieu Desnoyers Feb. 12, 2024, 4:30 p.m. UTC
Change the return value from NULL to PTR_ERR(-EOPNOTSUPP) for
CONFIG_DAX=n to be consistent with the fact that CONFIG_DAX=y
never returns NULL.

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@efficios.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.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: linux-arch@vger.kernel.org
Cc: linux-cxl@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-xfs@vger.kernel.org
Cc: dm-devel@lists.linux.dev
Cc: nvdimm@lists.linux.dev
---
 drivers/dax/super.c | 5 +++++
 include/linux/dax.h | 6 +-----
 2 files changed, 6 insertions(+), 5 deletions(-)

Comments

Lukas Wunner Feb. 13, 2024, 6:32 a.m. UTC | #1
On Mon, Feb 12, 2024 at 11:30:54AM -0500, Mathieu Desnoyers wrote:
> Change the return value from NULL to PTR_ERR(-EOPNOTSUPP) for
> CONFIG_DAX=n to be consistent with the fact that CONFIG_DAX=y
> never returns NULL.

All the callers of alloc_dax() only check for IS_ERR().

Doesn't this result in a change of behavior in all the callers?
Previously they'd ignore the NULL return value and continue,
now they'll error out.

Given that, seems dangerous to add a Fixes tag with a v4.0 commit
and thus risk regressing all stable kernels.

Thanks,

Lukas
Dan Williams Feb. 13, 2024, 7:07 p.m. UTC | #2
Lukas Wunner wrote:
> On Mon, Feb 12, 2024 at 11:30:54AM -0500, Mathieu Desnoyers wrote:
> > Change the return value from NULL to PTR_ERR(-EOPNOTSUPP) for
> > CONFIG_DAX=n to be consistent with the fact that CONFIG_DAX=y
> > never returns NULL.
> 
> All the callers of alloc_dax() only check for IS_ERR().
> 
> Doesn't this result in a change of behavior in all the callers?
> Previously they'd ignore the NULL return value and continue,
> now they'll error out.
> 
> Given that, seems dangerous to add a Fixes tag with a v4.0 commit
> and thus risk regressing all stable kernels.

Oh, good catch, yes that Fixes tag should be:

4e4ced93794a dax: Move mandatory ->zero_page_range() check in alloc_dax()

...as that was the one that updated the alloc_dax() calling convention
without fixing up the CONFIG_DAX=n case.

This is a pre-requisite for restoring DAX operation on ARM32.
Mathieu Desnoyers Feb. 13, 2024, 8:07 p.m. UTC | #3
On 2024-02-13 14:07, Dan Williams wrote:
> Lukas Wunner wrote:
>> On Mon, Feb 12, 2024 at 11:30:54AM -0500, Mathieu Desnoyers wrote:
>>> Change the return value from NULL to PTR_ERR(-EOPNOTSUPP) for
>>> CONFIG_DAX=n to be consistent with the fact that CONFIG_DAX=y
>>> never returns NULL.
>>
>> All the callers of alloc_dax() only check for IS_ERR().
>>
>> Doesn't this result in a change of behavior in all the callers?
>> Previously they'd ignore the NULL return value and continue,
>> now they'll error out.
>>
>> Given that, seems dangerous to add a Fixes tag with a v4.0 commit
>> and thus risk regressing all stable kernels.
> 
> Oh, good catch, yes that Fixes tag should be:
> 
> 4e4ced93794a dax: Move mandatory ->zero_page_range() check in alloc_dax()
> 
> ...as that was the one that updated the alloc_dax() calling convention
> without fixing up the CONFIG_DAX=n case.
> 
> This is a pre-requisite for restoring DAX operation on ARM32.

I'll change the Fixes tag in this commit to:

Fixes: 4e4ced93794a ("dax: Move mandatory ->zero_page_range() check in alloc_dax()")

for v6.

Thanks,

Mathieu
diff mbox series

Patch

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index 0da9232ea175..205b888d45bf 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)
 {
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)
 {