[PATCH v2 0/2] Add a seqcount between gup_fast and copy_page_range()

From: Jason Gunthorpe <jgg@nvidia.com>
To: <linux-kernel@vger.kernel.org>, Peter Xu <peterx@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>,
	Christoph Hellwig <hch@lst.de>, Hugh Dickins <hughd@google.com>,
	Jan Kara <jack@suse.cz>, Jann Horn <jannh@google.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Kirill Shutemov <kirill@shutemov.name>,
	Kirill Tkhai <ktkhai@virtuozzo.com>,
	Leon Romanovsky <leonro@nvidia.com>,
	Linux-MM <linux-mm@kvack.org>, Michal Hocko <mhocko@suse.com>,
	Oleg Nesterov <oleg@redhat.com>
Subject: [PATCH v2 0/2] Add a seqcount between gup_fast and copy_page_range()
Date: Fri, 30 Oct 2020 11:46:19 -0300	[thread overview]
Message-ID: <0-v2-dfe9ecdb6c74+2066-gup_fork_jgg@nvidia.com> (raw)

As discussed and suggested by Linus use a seqcount to close the small race
between gup_fast and copy_page_range().

Unfortunately the good suggestion to just use write_seqcount_begin() blows
up lockdep immediately due to the (new?) requirement that the write side
of seqcount be in a preempt disabled region. For this application it does
not seem like a good idea, nor is it necessary as we don't spin on retry.
This is solved by being the first place to use raw_write_seqcount_t_begin()

This can go after the merge window. I was table to test it using two
threads, one forking and the other using ibv_reg_mr() to trigger GUP
fast. Modifying copy_page_range() to sleep made the window large enough to
reliably hit to test the logic.

v2:
 - Use start not addr in lockless_pages_from_mm
 - Replace unsigned long casts with using the proper variable type
 - Update comments
 - Use raw_write_seqcount_t_begin() instead of open coding
 - Update commit messages
v1: https://lore.kernel.org/r/0-v1-281e425c752f+2df-gup_fork_jgg@nvidia.com

To: linux-kernel@vger.kernel.org
To: Peter Xu <peterx@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Linux-MM <linux-mm@kvack.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Kirill Shutemov <kirill@shutemov.name>
Cc: Hugh Dickins <hughd@google.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Jann Horn <jannh@google.com>

Jason Gunthorpe (2):
  mm: reorganize internal_get_user_pages_fast()
  mm: prevent gup_fast from racing with COW during fork

 arch/x86/kernel/tboot.c    |   1 +
 drivers/firmware/efi/efi.c |   1 +
 include/linux/mm_types.h   |   7 +++
 kernel/fork.c              |   1 +
 mm/gup.c                   | 118 +++++++++++++++++++++++--------------
 mm/init-mm.c               |   1 +
 mm/memory.c                |  10 +++-
 7 files changed, 93 insertions(+), 46 deletions(-)

-- 
2.28.0