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

On Mon, Aug 3, 2026, at 2:13 PM, Alison Schofield wrote:
> 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
> 

Thanks Alison, done in my local tree.

The patch that drops dev_dax_get() should hit your inbox today or tomorrow;
the one that adds dev_dax_find() will stay with this series.

Best,
John