mm: remove WARN_ON_ONCE() in file_has_valid_mmap_hooks()

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.

Link: https://lkml.kernel.org/r/20250514084024.29148-1-lorenzo.stoakes@oracle.com


Fixes: c84bf6dd ("mm: introduce new .mmap_prepare() file callback")
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202505141434.96ce5e5d-lkp@intel.com


Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>