linux-kernel - [PATCH v2 0/4] mm: Break COW for pinned pages during fork()

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Message-Id: <20200925222600.6832-1-peterx@redhat.com>
Date:   Fri, 25 Sep 2020 18:25:56 -0400
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     peterx@...hat.com, Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>, Yang Shi <shy828301@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>, Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Jan Kara <jack@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>
Subject: [PATCH v2 0/4] mm: Break COW for pinned pages during fork()

Due to the rebase to latest rc6, the major pte copy patch changed a lot.  So
maybe not that useful to write a changelog any more.  However all the comments
should be addressed as long as discussed in previous thread.  Please shoot if I
missed anything important.

This series is majorly inspired by the previous discussion on the list [1],
starting from the report from Jason on the rdma test failure.  Linus proposed
the solution, which seems to be a very nice approach to avoid the breakage of
userspace apps that didn't use MADV_DONTFORK properly before.  More information
can be found in that thread too.

I tested it myself with fork() after vfio pinning a bunch of device pages, and
I verified that the new copy pte logic worked as expected at least in the most
general path.  However I didn't test thp case yet because afaict vfio does not
support thp backed dma pages.  Luckily, the pmd/pud thp patch is much more
straightforward than the pte one, so hopefully it can be directly verified by
some code review plus some more heavy-weight rdma tests.

Patch 1:      Introduce mm.has_pinned
Patch 2:      Preparation patch
Patch 3:      Early cow solution for pte copy for pinned pages
Patch 4:      Same as above, but for thp (pmd/pud).

Hugetlbfs fix is still missing, but as planned, that's not urgent so we can
work upon.  Comments greatly welcomed.

[1] https://lore.kernel.org/lkml/20200914143829.GA1424636@nvidia.com/

Thanks.

Peter Xu (4):
  mm: Introduce mm_struct.has_pinned
  mm/fork: Pass new vma pointer into copy_page_range()
  mm: Do early cow for pinned pages during fork() for ptes
  mm/thp: Split huge pmds/puds if they're pinned when fork()

 include/linux/mm.h       |   2 +-
 include/linux/mm_types.h |  10 +++
 kernel/fork.c            |   3 +-
 mm/gup.c                 |   6 ++
 mm/huge_memory.c         |  28 ++++++
 mm/memory.c              | 186 ++++++++++++++++++++++++++++++++++-----
 6 files changed, 212 insertions(+), 23 deletions(-)

-- 
2.26.2