Re: [PATCH 0/3] ksm: fix incorrect count of merged pages when enabling use_zero_pages
From: David Hildenbrand
Date: Thu Sep 29 2022 - 13:53:46 EST
On 29.09.22 14:05, Claudio Imbrenda wrote:
On Thu, 29 Sep 2022 13:12:44 +0200
David Hildenbrand <david@xxxxxxxxxx> wrote:
On 29.09.22 12:36, Claudio Imbrenda wrote:
On Thu, 29 Sep 2022 11:21:44 +0200
David Hildenbrand <david@xxxxxxxxxx> wrote:
On 29.09.22 04:52, xu.xin.sc@xxxxxxxxx wrote:
From: xu xin <xu.xin16@xxxxxxxxxx>
Before enabling use_zero_pages by setting /sys/kernel/mm/ksm/
use_zero_pages to 1, pages_sharing of KSM is basically accurate. But
after enabling use_zero_pages, all empty pages that are merged with
kernel zero page are not counted in pages_sharing or pages_shared.
That is because the rmap_items of these ksm zero pages are not
appended to The Stable Tree of KSM.
We need to add the count of empty pages to let users know how many empty
pages are merged with kernel zero page(s).
Please see the subsequent patches for details.
Just raising the topic here because it's related to the KSM usage of the
shared zero-page:
MADV_UNMERGEABLE and other ways to trigger unsharing will *not* unshare
the shared zeropage as placed by KSM (which is against the
MADV_UNMERGEABLE documentation at least). It will only unshare actual
KSM pages. We might not want want to blindly unshare all shared
zeropages in applicable VMAs ... using a dedicated shared zero (KSM)
page -- instead of the generic zero page -- might be one way to handle
this cleaner.
I don't understand why do you need this.
first of all, one zero page would not be enough (depending on the
architecture, e.g. on s390x you need many). the whole point of zero
page merging is that one zero page is not enough.
I don't follow. Having multiple ones is a pure optimization on s390x (I
recall something about cache coloring), no? So why should we blindly
care in the special KSM use case here?
because merging pages full of zeroes with only one page will have
negative performance on those architectures that need cache colouring
(and s390 is not even the only architecture that needs it)
the whole point of merging pages full of zeroes with zero pages is to
not lose the cache colouring.
otherwise you could just let KSM merge all pages full of zeroes with
one page (which is what happens without use_zero_pages), and all the
numbers are correct.
if you are not on s390 or MIPS, you have no use for use_zero_pages
Ah, I see now that use_zero_pages is really only (mostly) s390x
specific. I already wondered why on earth we would really need that,
thanks for pointing that out.
One question I'd have is: why is the shared zero page treated special in
KSM then *at all*. Cache coloring problem should apply to *each and
every* deduplicated page.
Why is a page filled with 0xff any different from a page filled with 0x0?
Yes, I read e86c59b1b12d. It doesn't mention any actual performance
numbers and if the performance only applies to some microbenchmarks
nobody cares about.
Did you post some benchmarks results back then? That would be
interesting. I assume that the shared zeropage was simply the low
hanging fruit.
second, once a page is merged with a zero page, it's not really handled
by KSM anymore. if you have a big allocation, of which you only touch a
few pages, would the rest be considered "merged"? no, it's just zero
pages, right?
If you haven't touched memory, there is nothing populated -- no shared
zeropage.
We only populate shared zeropages in private anonymous mappings on read
access without prior write.
that's what I meant. if you read without writing, you get zero pages.
you don't consider those to be "shared" from a KSM point of view
does it make a difference if some pages that have been written to but
now only contain zeroes are discarded and mapped back to the zero pages?
That's a good question. When it comes to unmerging, you'd might expect
that whatever was deduplicated will get duplicated again -- and your
memory consumption will adjust accordingly. The stats might give an
admin an idea regarding how much memory is actually overcommited. See
below on the important case where we essentially never see the shared
zeropage.
The motivation behind these patches would be great -- what is the KSM
user and what does it want to achieve with these numbers?
this is the same, except that we take present pages with zeroes in it
and we discard them and map them to zero pages. it's kinda like if we
had never touched them.
MADV_UNMERGEABLE
"Undo the effect of an earlier MADV_MERGEABLE operation on the
specified address range; KSM unmerges whatever pages it had merged in
the address range specified by addr and length."
Now please explain to me how not undoing a zeropage merging is correct
according to this documentation.
because once it's discarded and replaced with a zero page, the page is
not handled by KSM anymore.
I understand what you mean, that KSM did an action that now cannot be
undone, but how would you differentiate between zero pages that were
never written to and pages that had been written to and then discarded
and mapped back to a zero page because they only contained zeroes?
An application that always properly initializes (write at least some
part once) all its memory will never have the shared zeropage mapped. VM
guest memory comes to mind, probably still the most important KSM use case.
There are currently some remaining issues when taking a GUP R/O longterm
pin on such a page (e.g., vfio). In contrast to KSM pages, such pins are
not reliable for the shared zeropage, but I have fixes for them pending.
However, that is rather a corner case (it didn't work at all correctly a
while ago) and will be sorted out soon.
So the question is if MADV_UNMERGEABLE etc. (stats) should be adjusted
to document the behavior with use_zero_pages accordingly.
--
Thanks,
David / dhildenb