linux-kernel - [PATCH mm-stable RFC 0/2] mm/nommu: don't use VM_MAYSHARE for MAP

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Date:   Mon, 19 Dec 2022 17:30:11 +0100
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        io-uring@...r.kernel.org, David Hildenbrand <david@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nicolas Pitre <nico@...xnic.net>, Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>
Subject: [PATCH mm-stable RFC 0/2] mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings

Trying to reduce the confusion around VM_SHARED and VM_MAYSHARE first
requires !CONFIG_MMU to stop using VM_MAYSHARE for MAP_PRIVATE mappings.
CONFIG_MMU only sets VM_MAYSHARE for MAP_SHARED mappings.

This paves the way for further VM_MAYSHARE and VM_SHARED cleanups: for
example, renaming VM_MAYSHARED to VM_MAP_SHARED to make it cleaner what
is actually means.

Let's first get the weird case out of the way and not use VM_MAYSHARE in
MAP_PRIVATE mappings, using a new VM_MAYOVERLAY flag instead.

I am not a NOMMU expert, but my basic testing with risc64-nommu with
buildroot under QEMU revealed no surprises.

Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Nicolas Pitre <nico@...xnic.net>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Pavel Begunkov <asml.silence@...il.com>

David Hildenbrand (2):
  mm/nommu: factor out check for NOMMU shared mappings into
    is_nommu_shared_mapping()
  mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings

 drivers/char/mem.c    |  2 +-
 fs/cramfs/inode.c     |  2 +-
 fs/proc/task_nommu.c  |  2 +-
 fs/ramfs/file-nommu.c |  2 +-
 fs/romfs/mmap-nommu.c |  2 +-
 include/linux/mm.h    | 20 ++++++++++++++
 io_uring/io_uring.c   |  2 +-
 mm/nommu.c            | 62 +++++++++++++++++++++++++------------------
 8 files changed, 62 insertions(+), 32 deletions(-)

-- 
2.38.1