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

Michal Hocko
2019-05-14T14:41:09Z

[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 [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.

Please make sure to describe a usecase that warrants adding a new
interface we have to maintain for ever.

>
> 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(-)
>
> --
> 2.21.0
>

--
Michal Hocko
SUSE Labs