Re: [PATCH v6 0/6] ksm: support tracking KSM-placed zero-pages

[Claudio Imbrenda]

On Mon, 13 Mar 2023 14:03:33 +0100
David Hildenbrand <david@xxxxxxxxxx> wrote:

> On 10.02.23 02:15, yang.yang29@xxxxxxxxxx wrote:
> > From: xu xin <xu.xin16@xxxxxxxxxx>
> >   
> 
> Hi,
> 
> sorry for the late follow-up. Still wrapping my head around this and 
> possible alternatives. I hope we'll get some comments from others as 
> well about the basic approach.
> 
> > The core idea of this patch set is to enable users to perceive the number of any
> > pages merged by KSM, regardless of whether use_zero_page switch has been turned
> > on, so that users can know how much free memory increase is really due to their
> > madvise(MERGEABLE) actions. But the problem is, when enabling use_zero_pages,
> > all empty pages will be merged with kernel zero pages instead of with each
> > other as use_zero_pages is disabled, and then these zero-pages are no longer
> > monitored by KSM.
> > 
> > The motivations for me to do this contains three points:
> > 
> > 1) 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); see the link:
> >     https://lore.kernel.org/lkml/4a3daba6-18f9-d252-697c-197f65578c44@xxxxxxxxxx/
> > 
> > 2) We cannot know how many pages are zero pages placed by KSM when
> >     enabling use_zero_pages, which hides the critical information about
> >     how much actual memory are really saved by KSM. Knowing how many
> >     ksm-placed zero pages are helpful for user to use the policy of madvise
> >     (MERGEABLE) better because they can see the actual profit brought by KSM.
> > 
> > 3) The zero pages placed-by KSM are different from those initial empty page
> >     (filled with zeros) which are never touched by applications. The former
> >     is active-merged by KSM while the later have never consume actual memory.
> >   
> 
> I agree with all of the above, but it's still unclear to me if there is 
> a real downside to a simpler approach:
> 
> (1) Tracking the shared zeropages. That would be fairly easy: whenever
>      we map/unmap a shared zeropage, we simply update the counter.
> 
> (2) Unmerging all shared zeropages inside the VMAs during
>      MADV_UNMERGEABLE.
> 
> (3) Documenting that MADV_UNMERGEABLE will also unmerge the shared
>      zeropage when toggle xy is flipped.
> 
> It's certainly simpler and doesn't rely on the rmap item. See below.

I would surely prefer a simpler approach

> 
> > use_zero_pages is useful, not only because of cache colouring as described
> > in doc, but also because use_zero_pages can accelerate merging empty pages
> > when there are plenty of empty pages (full of zeros) as the time of
> > page-by-page comparisons (unstable_tree_search_insert) is saved. So we hope to
> > implement the support for ksm zero page tracking without affecting the feature
> > of use_zero_pages.
> > 
> > Zero pages may be the most common merged pages in actual environment(not only VM but
> > also including other application like containers). Enabling use_zero_pages in the
> > environment with plenty of empty pages(full of zeros) will be very useful. Users and
> > app developer can also benefit from knowing the proportion of zero pages in all
> > merged pages to optimize applications.
> >   
> 
> I agree with that point, especially after I read in a paper that KSM 
> applied to some applications mainly deduplicates pages filled with 0s. 
> So it seems like a reasonable thing to optimize for.
> 
> > With the patch series, we can both unshare zero-pages(KSM-placed) accurately
> > and count ksm zero pages with enabling use_zero_pages.  
> 
> The problem with this approach I see is that it fundamentally relies on 
> the rmap/stable-tree to detect whether a zeropage was placed or not.
> 
> I was wondering, why we even need an rmap item *at all* anymore. Why 
> can't we place the shared zeropage an call it a day (remove the rmap 
> item)? Once we placed a shared zeropage, the next KSM scan should better 
> just ignore it, it's already deduplicated.
> 
> So if most pages we deduplicate are shared zeropages, it would be quite 
> interesting to reduce the memory overhead and avoid rmap items, instead 
> of building new functionality on top of it?
> 
> 
> 
> If we'd really want to identify whether a zeropage was deduplciated by 
> KSM, we could try storing that information inside the PTE instead of 

this is interesting, but needs caution, for the reason you mention below

> inside the RMAP. Then, we could directly adjust the counter when zapping 
> the shared zeropage or during MADV_DONTNEED/when unmerging.
> 
> Eventually, we could simply say that
> * !pte_dirty(): zeropage placed during fault
> * pte_dirty(): zeropage placed by KSM
> 
> Then it would also be easy to adjust counters and unmerge. We'd limit 
> this handling to known-working architectures initially (spec64 still has 
> the issue that pte_mkdirty() will set a pte writable ... and my patch to 
> fix that was not merged yet). We'd have to double-check all 
> pte_mkdirty/pte_mkclean() callsites.

this will be... interesting

>