linux-kernel - Re: [PATCH mm-new v6 2/5] mm: khugepaged: refine scan progress number

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Message-ID: <4f719bed-89bf-44f3-a1cc-39ddc7c66824@arm.com>
Date: Thu, 5 Feb 2026 17:37:57 +0530
From: Dev Jain <dev.jain@....com>
To: Vernon Yang <vernon2gm@...il.com>,
 "David Hildenbrand (arm)" <david@...nel.org>
Cc: akpm@...ux-foundation.org, lorenzo.stoakes@...cle.com, ziy@...dia.com,
 baohua@...nel.org, lance.yang@...ux.dev, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Vernon Yang <yanglincheng@...inos.cn>
Subject: Re: [PATCH mm-new v6 2/5] mm: khugepaged: refine scan progress number


On 05/02/26 11:38 am, Vernon Yang wrote:
> On Thu, Feb 5, 2026 at 5:35 AM David Hildenbrand (arm) <david@...nel.org> wrote:
>> [...]
>>
>>> +     if (cur_progress) {
>>> +             if (_pte >= pte + HPAGE_PMD_NR)
>>> +                     *cur_progress = HPAGE_PMD_NR;
>>> +             else
>>> +                     *cur_progress = _pte - pte + 1;
>> *cur_progress = max(_pte - pte + 1, HPAGE_PMD_NR);
> I guess, your meaning is "min(_pte - pte + 1, HPAGE_PMD_NR)", not max().
>
>> ?
>>
>> It's still a bit nasty, though.
>>
>> Can't we just add one at the beginning of the loop and let the compiler
>> optimize that? ;)
> I'm also worried that the compiler can't optimize this since the body of
> the loop is complex, as with Dev's opinion [1].
>
> [1] https://lore.kernel.org/linux-mm/7c4b5933-7bbd-4ad7-baef-830304a09485@arm.com
>
> If you have a strong recommendation for this, please let me know, Thanks!

I haven't explicitly checked with assembly, but I am fairly sure this won't get optimized.
There are two cases where it could have been optimized:

1) Had the compiler inlined hpage_collapse_scan_pmd
2) Had the compiler done something like
   if (p) -> foo(), where foo() contains the complete for loop, with the increment
   else -> bar(), where bar() contains the complete for loop, without the increment

Both of which are highly unlikely because of the complexity of the function.

>
>>> +     }
>>>       pte_unmap_unlock(pte, ptl);
>>>       if (result == SCAN_SUCCEED) {
>>>               result = collapse_huge_page(mm, start_addr, referenced,
>>> @@ -2286,8 +2301,9 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
>>>       return result;
>>>   }
>>>
>>> -static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
>>> -             struct file *file, pgoff_t start, struct collapse_control *cc)
>>> +static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm,
>>> +             unsigned long addr, struct file *file, pgoff_t start,
>>> +             unsigned int *cur_progress, struct collapse_control *cc)
>>>   {
>>>       struct folio *folio = NULL;
>>>       struct address_space *mapping = file->f_mapping;
>>> @@ -2376,6 +2392,8 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
>>>                       cond_resched_rcu();
>>>               }
>>>       }
>>> +     if (cur_progress)
>>> +             *cur_progress = max(xas.xa_index - start, 1UL);
>> I would really just keep it simple here and do a
>>
>> *cur_progress = HPAGE_PMD_NR;
>>
>> This stuff is hard to reason about, so I would just leave the file case
>> essentially unchanged.
>>
>> IIRC, it would not affect the numbers you report in the patch description?
> Yes, Let's keep it simple, always equal to HPAGE_PMD_NR in file case.
>
> --
> Thanks,
> Vernon