Re: [PATCH mm-new v3 1/3] mm/khugepaged: optimize PTE scanning with if-else-if-else-if chain

[Lance Yang]

On 2025/10/15 17:16, Lorenzo Stoakes wrote:
> On Wed, Oct 15, 2025 at 12:49:26PM +0800, Lance Yang wrote:
> > On 2025/10/14 20:27, David Hildenbrand wrote:
> > > On 14.10.25 14:17, Lorenzo Stoakes wrote:
> > > > On Wed, Oct 08, 2025 at 12:37:46PM +0800, Lance Yang wrote:
> > > > > From: Lance Yang <lance.yang@xxxxxxxxx>
> > > > >
> > > > > As pointed out by Dev, the PTE checks for disjoint conditions in the
> > > > > scanning loops can be optimized. is_swap_pte, (pte_none && is_zero_pfn),
> > > > > and pte_uffd_wp are mutually exclusive.
> > > >
> > > > But you're not using is_swap_pte anywhere :) This comes back to my review
> > > > quesiotn on the series this is dependent upon.
> > > >
> > > > > This patch refactors the loops in both
> > > > > __collapse_huge_page_isolate() and
> > > > > hpage_collapse_scan_pmd() to use a continuous if-else-if-else-if chain
> > > > > instead of separate if blocks. While at it, the redundant pte_present()
> > > > > check before is_zero_pfn() is also removed.
> > > >
> > > > I mean see review below, I don't see why you're doing this and I am
> > > > unconvinced by how redundant that check is.
> >
> > Ah, good catch! Lorenzo, thanks!!!
> >
> > > > Also this just feels like it should be part of the series where you
> > > > change
> > > > these? I'm not sure why this is separate?
> > >
> > > I think Lance is trying to unify both scanning functions to look alike,
> > > such that when he refactors them out in patch #3 it looks more straight
> > > forward.
> > >
> > > The missing pte_present() check in hpage_collapse_scan_pmd() is interesting
> >
> > Yep, indeed ;)
> >
> > > Likely there is one such check missing there?
> >
> > I think the risk is exactly how pte_pfn() would handle a swap PTE ...
> >
> > A swap PTE contains completely different data(swap type and offset).
> > pte_pfn() doesn't know this, so if we feed a swap entry to it, it will
> > spit out a junk PFN :)
> >
> > What if that junk PFN happens to match the zeropage's PFN by sheer
> > chance? IHMO, it's really unlikely, but it would be really bad if it did.
> >
> > Clearly, pte_present() prevents this ;)
>
> Yeah, not so clearly kinda the whole point I'm making here. I mean all this code
> sucks because we have deeply nested conditions and you have to keep in your mind
> that 'oh we already checked for X so we can do Y'.

I see :)

> But the THP code is horrible in general.

Yep, you'll get no argument from me on that one ;p

The code is indeed tricky ...

> Anyway let's stay focused (so I can stand a chance of catching up with my
> post-vacation backlog :), I will check the respin once you send!

Cheers!

This series has been dropped from mm-new. I'm going to take a bit to
rethink the approach based on the feedback.

> > By the way, I noticed there are other places in khugepaged.c that
> > call pte_pfn() without being under a pte_present() check.
> >
> > Perhaps those should all be fixed as well in a separate patch?
>
> Yeah I'm not surprised and sure indeed.
>
> Thanks, Lorenzo

In the meantime, I'll send up a separate patch for the missing
pte_present() checks first ;)

Thanks,
Lance