linux-kernel - Re: [PATCH mm-new v7 4/5] mm: khugepaged: skip lazy-free folios

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Message-ID: <bfda8f27-cd2d-43dd-9fdc-cebf7b7cd7f8@kernel.org>
Date: Sat, 7 Feb 2026 23:25:27 +0100
From: "David Hildenbrand (Arm)" <david@...nel.org>
To: Barry Song <21cnbao@...il.com>
Cc: Lance Yang <lance.yang@...ux.dev>, Vernon Yang <vernon2gm@...il.com>,
 akpm@...ux-foundation.org, lorenzo.stoakes@...cle.com, ziy@...dia.com,
 dev.jain@....com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Vernon Yang <yanglincheng@...inos.cn>
Subject: Re: [PATCH mm-new v7 4/5] mm: khugepaged: skip lazy-free folios

On 2/7/26 23:17, Barry Song wrote:
> On Sun, Feb 8, 2026 at 6:05 AM David Hildenbrand (Arm) <david@...nel.org> wrote:
>>
>> On 2/7/26 23:01, Barry Song wrote:
>>>
>>> Maybe change “just not skip” to “just skip”?
>>>
>>> If the goal is to avoid the collapse overhead for folios that are
>>> about to be dropped, we might consider skipping collapse for the
>>> entire VMA？
>> If there is no memory pressure in the system, why wouldn't you just want
>> to collapse in a VM_DROPPABLE region?
>>
>> "about to be dropped" only applies once there is actual memory pressure.
>> If not, these pages stick around forever.
> 
>   agree. But this brings us back to the philosophy of the original patch.
> If there is no memory pressure, lazyfree folios won’t be dropped, so
> collapsing them might also be reasonable.

It's about memory pressure in the future.

> 
> Just collapsing fully lazyfree folios with VM_DROPPABLE while
> skipping partially lazyfree VMAs seems a bit confusing to me :-)

Think of it like this:

All folios in VM_DROPPABLE are lazyfree. Collapsing maintains that 
property. So you can just collapse and memory pressure in the future 
will free it up.

In contrast, collapsing in !VM_DROPPABLE does not maintain that 
property. The collapsed folio will not be lazyfree and memory pressure 
in the future will not be able to free it up.

-- 
Cheers,

David