[PATCH RFC v2 0/4] mm/ksm: add option to automerge VMAs

[Oleksandr Natalenko <oleksandr@redhat.com>]

By default, KSM works only on memory that is marked by madvise(). And the
only way to get around that is to either:

  * use LD_PRELOAD; or
  * patch the kernel with something like UKSM or PKSM.

Instead, lets implement a sysfs knob, which allows marking VMAs as
mergeable. This can be used manually on some task in question or by some
small userspace helper daemon.

The knob is named "force_madvise", and it is write-only. It accepts a PID
to act on. To mark the VMAs as mergeable, use:

   # echo PID > /sys/kernel/mm/ksm/force_madvise

To unmerge all the VMAs, use the same approach, prepending the PID with
the "minus" sign:

   # echo -PID > /sys/kernel/mm/ksm/force_madvise

This patchset is based on earlier Timofey's submission [1], but it doesn't
use dedicated kthread to walk through the list of tasks/VMAs. Instead,
it is up to userspace to traverse all the tasks in /proc if needed.

The previous suggestion [2] was based on amending do_anonymous_page()
handler to implement fully automatic mode, but this approach was
incorrect due to improper locking and not desired due to excessive
complexity.

The current approach just implements minimal interface and leaves the
decision on how and when to act to userspace.

Thanks.

[1] https://lore.kernel.org/patchwork/patch/1012142/
[2] http://lkml.iu.edu/hypermail/linux/kernel/1905.1/02417.html

Oleksandr Natalenko (4):
  mm/ksm: introduce ksm_enter() helper
  mm/ksm: introduce ksm_leave() helper
  mm/ksm: introduce force_madvise knob
  mm/ksm: add force merging/unmerging documentation

 Documentation/admin-guide/mm/ksm.rst |  11 ++
 mm/ksm.c                             | 160 +++++++++++++++++++++------
 2 files changed, 137 insertions(+), 34 deletions(-)

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2.21.0