[PATCH] mm: remove WARN_ON_ONCE() in file_has_valid_mmap_hooks()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

Having encountered a trinity report in linux-next (Linked in the 'Closes'
tag) it appears that there are legitimate situations where a file-backed
mapping can be acquired but no file->f_op->mmap or file->f_op->mmap_prepare
is set, at which point do_mmap() should simply error out with -ENODEV.

Since previously we did not warn in this scenario and it appears we rely
upon this, restore this situation, while retaining a WARN_ON_ONCE() for the
case where both are set, which is absolutely incorrect and must be
addressed and thus always requires a warning.

If further work is required to chase down precisely what is causing this,
then we can later restore this, but it makes no sense to hold up this
series to do so, as this is existing and apparently expected behaviour.

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202505141434.96ce5e5d-lkp@intel.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---

Andrew -

Since this series is in mm-stable we should take this fix there asap (and
certainly get it to -next to fix any further error reports). I didn't know
whether it was best for it to be a fix-patch or not, so have sent
separately so you can best determine what to do with it :)

Thanks, Lorenzo

 include/linux/fs.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index e2721a1ff13d..09c8495dacdb 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2248,7 +2248,7 @@ static inline bool file_has_valid_mmap_hooks(struct file *file)
 	/* Hooks are mutually exclusive. */
 	if (WARN_ON_ONCE(has_mmap && has_mmap_prepare))
 		return false;
-	if (WARN_ON_ONCE(!has_mmap && !has_mmap_prepare))
+	if (!has_mmap && !has_mmap_prepare)
 		return false;

 	return true;
--
2.49.0