Re: [PATCH V12 01/12] dax: replace exported dax_dev_get() with non-allocating dax_dev_find()

On Mon, Aug 03, 2026 at 02:28:26AM +0000, John Groves wrote:
> From: John Groves <[email protected]>
> 
> This fix is in response to a Sashiko review, and some subsequent
> analysis.
> 
> dax_dev_get() uses iget5_locked() which creates a new inode if no
> matching one exists. This is correct for the internal caller
> (alloc_dax), but dangerous for external callers that look up devices
> from user-supplied or metadata-supplied dev_t values:
> 
> 1. A new inode is created with DAXDEV_ALIVE set but no backing driver,
>    no ops, and no IDA-allocated minor number.
> 
> 2. On teardown, dax_destroy_inode() warns because kill_dax() was never
>    called, and dax_free_inode() calls ida_free() for a minor that was
>    never ida_alloc'd -- potentially freeing the minor of a real device.
> 
> Add dax_dev_find() which uses ilookup5() for lookup-only semantics:
> it returns an existing dax_device with an elevated inode reference, or
> NULL if no device with the given dev_t exists. It never creates inodes.
> A dax_alive() check under dax_read_lock() guards against returning a
> device that is concurrently being torn down by kill_dax().
> 
> Make dax_dev_get() static again (internal to super.c for alloc_dax),
> export dax_dev_find() instead, and update the two external callers
> (famfs_inode.c, famfs.c). Also add the missing CONFIG_DAX=n stub.

There are no external callers yet as those arrive in subsequent
famfs patches.


> 
> About the 'fixes' tag: this removes the export of dax_dev_get(),
> which was flawed, and replaces is with dax_dev_find(). It feels like
> the fixes tag makes sense for correcting an ABI error.
> 
> Fixes: 2ae624d5a555d ("dax: export dax_dev_get()")

Hi John,

I think this should be split.

Please send a standalone DAX patch that only removes the dax_dev_get()
export (make it static again, drop the header declaration). It's unused
in-tree and unsafe for its intended use, so it stands on its own with
no FAMFS dependency. I'll take it through the DAX tree for 7.3.

Please drop the Fixes: tag on the removal. IIUC the stable team uses
it to pick backports, and this shouldn't land in 7.2.y. Removing this
fixes nothing since no in-tree code calls the symbol. Name the commit
in prose instead, something like:

  Commit 2ae624d5a555 ("dax: export dax_dev_get()") exported
  dax_dev_get() in v7.2 for famfs, which has not merged. The export
  has never had an in-tree caller, so make dax_dev_get() static again.

Keep the dax_dev_find() addition in this famfs series, so the new export
lands with famfs.

-- Alison

> 
> Reviewed-by: Dave Jiang <[email protected]>
> Reviewed-by: Alison Schofield <[email protected]>

I think you can carry the tags for both patches because the code
should end up byte identical.

snip