Re: [PATCH RFC v2 0/4] mm/ksm: add option to automerge VMAs
From: Michal Hocko
Date: Tue May 14 2019 - 10:53:05 EST
On Tue 14-05-19 16:41:05, Michal Hocko wrote:
> [This is adding a new user visible interface so you should be CCing
> linux-api mailing list. Also CC Hugh for KSM in general. Done now]
> On Tue 14-05-19 15:16:50, Oleksandr Natalenko wrote:
> > 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 , 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  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.
> Please make sure to describe a usecase that warrants adding a new
> interface we have to maintain for ever.
> > Thanks.
> >  https://lore.kernel.org/patchwork/patch/1012142/
> >  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(-)
> > --
> > 2.21.0
> Michal Hocko
> SUSE Labs