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

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Message-ID: <93ec17e6-e39a-4575-a592-0a1d6c47b30a@kernel.org>
Date: Thu, 5 Feb 2026 13:28:25 +0100
From: "David Hildenbrand (Arm)" <david@...nel.org>
To: Dev Jain <dev.jain@....com>, Vernon Yang <vernon2gm@...il.com>
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 2/5/26 13:07, Dev Jain wrote:
> 
> 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:
>>> [...]
>>>
>>> *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

Yeah, there are two callers so that likely does not happen.

> 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.

Not sure if the compiler would be to optimize this out also in 
non-inlined cases. In any case, I wonder if this must be optimized at 
all ...

-- 
Cheers,

David