Re: [PATCH mm-new 1/3] mm/khugepaged: skip unsuitable VMAs earlier in khugepaged_scan_mm_slot()

[Lance Yang]

On 2025/9/16 17:58, Lorenzo Stoakes wrote:
> On Tue, Sep 16, 2025 at 10:48:22AM +0100, Kiryl Shutsemau wrote:
> > On Tue, Sep 16, 2025 at 10:39:53AM +0100, Lorenzo Stoakes wrote:
> > > On Tue, Sep 16, 2025 at 10:29:11AM +0100, Kiryl Shutsemau wrote:
> > > > On Tue, Sep 16, 2025 at 02:21:26PM +0800, Lance Yang wrote:
> > > > > Users of mlock() expect low and predictable latency. THP collapse is a
> > > > > heavy operation that introduces exactly the kind of unpredictable delays
> > > > > they want to avoid. It has to unmap PTEs, copy data from the small folios
> > > > > to a new THP, and then remap the THP back to the PMD ;)
> > > >
> > > > Generally, we allow minor page faults into mlocked VMAs and avoid major.
> > > > This is minor page fault territory in my view.
> > >
> > > Hm, but we won't be causing minor faults via reclaim right, since they're
> > > not on any LRU?
> >
> > PTEs are still present when we do THP allocation. No reclaim while the
> > access is blocked. We only block the access on copy and PTEs->PMD
> > collapse.
>
> Right indeed, esp. with compaction being allowed for mlock, I agree with you
> that this patch should be dropped :)

Got it. Will do ;)

Thanks,
Lance