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: <80771030-0d8d-4f65-803a-f89da3b59796@linux.dev>
Date: Wed, 15 Oct 2025 12:49:26 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: 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 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 ;)

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?