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

From: Kirill Tkhai
Date: Mon May 13 2019 - 06:40:40 EST

Hi, Oleksandr,

On 10.05.2019 10:21, 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 so-called "always" mode, which allows marking
> VMAs as mergeable on do_anonymous_page() call automatically.
> The submission introduces a new sysctl knob as well as kernel cmdline option
> to control which mode to use. The default mode is to maintain old
> (madvise-based) behaviour.
> Due to security concerns, this submission also introduces VM_UNMERGEABLE
> vmaflag for apps to explicitly opt out of automerging. Because of adding
> a new vmaflag, the whole work is available for 64-bit architectures only.
>> 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.
> For my laptop it saves up to 300 MiB of RAM for usual workflow (browser,
> terminal, player, chats etc). Timofey's submission also mentions
> containerised workload that benefits from automerging too.

This all approach looks complicated for me, and I'm not sure the shown profit
for desktop is big enough to introduce contradictory vma flags, boot option
and advance page fault handler. Also, 32/64bit defines do not look good for
me. I had tried something like this on my laptop some time ago, and
the result was bad even in absolute (not in memory percentage) meaning.
Isn't LD_PRELOAD trick enough to desktop? Your workload is same all the time,
so you may statically insert correct preload to /etc/profile and replace
your mmap forever.

Speaking about containers, something like this may have a sense, I think.
The probability of that several containers have the same pages are higher,
than that desktop applications have the same pages; also LD_PRELOAD for
containers is not applicable.

But 1)this could be made for trusted containers only (are there similar
issues with KSM like with hardware side-channel attacks?!); 2) the most
shared data for containers in my experience is file cache, which is not
supported by KSM.

There are good results by the link [1], but it's difficult to analyze
them without knowledge about what happens inside them there.

Some of tests have "VM" prefix. What the reason the hypervisor don't mark
their VMAs as mergeable? Can't this be fixed in hypervisor? What is the
generic reason that VMAs are not marked in all the tests?

In case of there is a fundamental problem of calling madvise, can't we
just implement an easier workaround like a new write-only file:

#echo $task > /sys/kernel/mm/ksm/force_madvise

which will mark all anon VMAs as mergeable for a passed task's mm?

A small userspace daemon may write mergeable tasks there from time to time.

Then we won't need to introduce additional vm flags and to change
anon pagefault handler, and the changes will be small and only
related to mm/ksm.c, and good enough for both 32 and 64 bit machines.