mm: add documentation for the mmap_prepare file operation callback

This documentation makes it easier for a driver/file system implementer to
correctly use this callback.

It covers the fundamentals, whilst intentionally leaving the less lovely
possible actions one might take undocumented (for instance - the
success_hook, error_hook fields in mmap_action).

The document also covers the new VMA flags implementation which is the
only one which will work correctly with mmap_prepare.

Link: https://lkml.kernel.org/r/3aebf918c213fa2aecf00a31a444119b5bdd7801.1774045440.git.ljs@kernel.org
Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Bodo Stroesser <bostroesser@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Clemens Ladisch <clemens@ladisch.de>
Cc: David Hildenbrand <david@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Long Li <longli@microsoft.com>
Cc: Marc Dionne <marc.dionne@auristor.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Richard Weinberger <richard@nod.at>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/filesystems/index.rst

@@ -29,6 +29,7 @@ algorithms work.
    fiemap
    files
    locks
+   mmap_prepare
    multigrain-ts
    mount_api
    quota