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

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Message-ID: <cc998aee-2a98-45ae-875a-9712d4c29240@lucifer.local>
Date: Wed, 15 Oct 2025 10:16:03 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
        ziy@...dia.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
        npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
        baohua@...nel.org, ioworker0@...il.com, richard.weiyang@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mm-new v3 1/3] mm/khugepaged: optimize PTE scanning with
 if-else-if-else-if chain

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@...ux.dev>
> > > >
> > > > 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'.

But the THP code is horrible in general.

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!

>
> 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