Re: [PATCH v2 1/1] mm: skip mlocked THPs that are underused early in deferred_split_scan()

Kiryl Shutsemau
2025-09-08T12:45:10Z

On Mon, Sep 08, 2025 at 02:04:05PM +0200, David Hildenbrand wrote:
> On 08.09.25 13:44, Kiryl Shutsemau wrote:
> > On Mon, Sep 08, 2025 at 01:32:05PM +0200, David Hildenbrand wrote:
> > > On 08.09.25 12:38, Kiryl Shutsemau wrote:
> > > > On Mon, Sep 08, 2025 at 05:07:41PM +0800, Lance Yang wrote:
> > > > > From: Lance Yang <lance.yang@linux.dev>
> > > > >
> > > > > When we stumble over a fully-mapped mlocked THP in the deferred shrinker,
> > > > > it does not make sense to try to detect whether it is underused, because
> > > > > try_to_map_unused_to_zeropage(), called while splitting the folio, will not
> > > > > actually replace any zeroed pages by the shared zeropage.
> > > >
> > > > It makes me think, does KSM follows the same logic as
> > > > try_to_map_unused_to_zeropage()?
> > > >
> > > > I cannot immediately find what prevents KSM from replacing zeroed mlocked
> > > > folio with ZERO_PAGE().
> > > >
> > > > Hm?
> > >
> > > I assume if you're using mlock and at the same time enable KSM for a
> > > process/VMA, you're doing something wrong.
> > >
> > > In contrast, THP is supposed to be transparent (yeah, I know ...).
> >
> > Yeah, I guess it is user error.
> >
> > Maybe we should make ksm_compatible() return false for VM_LOCKED?
> > KSM breaks mlock() contract.
>
> I was thinking the same and falsely remembered that we would already be
> checking for that.
>
> >
> > But it can be risky if someone already relies on this broken behaviour.
>
> Could be.
>
> Staring at QEMU, we have the following parameters:
>
> mem-merge=on|off
>
> Enables or disables memory merge support. This feature, when
> supported by the host, de-duplicates identical memory pages
> among VMs instances (enabled by default).
>
> And
>
> -overcommit mem-lock=on|off|on-fault
>
> "Run qemu with hints about host resource overcommit. The default
> is to assume that host overcommits all resources."
>
>
> Now, I would assume that anybody who sets "-overcommit mem-lock=on" either
>
> (a) Has KSM disabled on that machine.
>
> (b) Sets mem-merge=off
>
> as well. But QEMU would allow for configuring it.

ksm_madvise(MADV_MERGEABLE) succeeds on !vma_ksm_compatible(), so it
wouldn't be functional breakage, but may result in unexpected increase
of memory consumption.

> Interestingly, mm_populate()->populate_vma_page_range() wants to break COW.
> [*]
>
> But if the app later calls fork(), we still allow for cow-sharing pages with
> the child. (another case of "don't do it", like KSM I guess)

CoW has bunch of these "don't do it". :P

> [*] it doesn't do it for mappings that start out R/O. I think we might end
> up with sharedzero pages in that case, but not sure if worth fixing.
>
> --
> Cheers
>
> David / dhildenb
>

--
Kiryl Shutsemau / Kirill A. Shutemov