[PATCH 0/4] mm: Simplfy cow handling

[Peter Xu <peterx@redhat.com>]

This is a small series that I picked up from Linus's suggestion [0] to simplify\r
cow handling (and also more strict) by checking against page refcounts rather\r
than mapcounts.\r
\r
I'm CCing the author and reviewer of commit 52d1e606ee73 on ksm ("mm: reuse\r
only-pte-mapped KSM page in do_wp_page()", 2019-03-05).  Please shoot if\r
there's any reason to keep the logic, or it'll be removed in this series.  For\r
more information, please refer to [3,4].\r
\r
The new mm counter in the last patch can be seen as RFC, depending on whether\r
anyone dislikes it... I used it majorly for observing the page reuses, so it is\r
kind of optional.\r
\r
Two tests I did:\r
\r
  - Run a busy loop dirty program [1] that uses 6G of memory, restrict to 1G\r
    RAM + 5G swap (cgroup).  A few hours later, all things still look good.\r
    Make sure to observe (still massive) correct page reuses using the new\r
    counter using the last patch, probably when swapping in.\r
\r
  - Run umapsort [2] to make sure uffd-wp will work again after applying this\r
    series upon master 5.9-rc1 (5.9-rc1 is broken).\r
\r
In all cases, I must confess it's quite pleased to post a series with diffstat\r
like this...  Hopefully this won't break anyone but only to make everything\r
better.\r
\r
Please review, thanks.\r
\r
[0] https://lore.kernel.org/lkml/CAHk-=wjn90-=s6MBerxTuP=-FVEZtR-LpoH9eenEQ3A-QfKTZw@mail.gmail.com\r
[1] https://github.com/xzpeter/clibs/blob/master/bsd/mig_mon/mig_mon.c\r
[2] https://github.com/LLNL/umap-apps/blob/develop/src/umapsort/umapsort.cpp\r
[3] https://lore.kernel.org/lkml/CAHk-=wh0syDtNzt9jGyHRV0r1pVX5gkdJWdenwmvy=dq0AL5mA@mail.gmail.com\r
[4] https://lore.kernel.org/lkml/CAHk-=wj5Oyg0LeAxSw_vizerm=sLd=sHfcVecZMKPZn6kNbbXA@mail.gmail.com\r
\r
Linus Torvalds (1):\r
  mm: Trial do_wp_page() simplification\r
\r
Peter Xu (3):\r
  mm/ksm: Remove reuse_ksm_page()\r
  mm/gup: Remove enfornced COW mechanism\r
  mm: Add PGREUSE counter\r
\r
 drivers/gpu/drm/i915/gem/i915_gem_userptr.c |  8 ---\r
 include/linux/ksm.h                         |  7 ---\r
 include/linux/vm_event_item.h               |  1 +\r
 mm/gup.c                                    | 40 ++------------\r
 mm/huge_memory.c                            |  7 +--\r
 mm/ksm.c                                    | 25 ---------\r
 mm/memory.c                                 | 60 +++++++--------------\r
 mm/vmstat.c                                 |  1 +\r
 8 files changed, 29 insertions(+), 120 deletions(-)\r
\r
-- \r
2.26.2\r
\r