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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <bd36d92d-dbf1-4818-bb31-0330c1f53577@kernel.org>
Date: Sat, 7 Feb 2026 23:05:44 +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 23:01, Barry Song wrote:
> On Sun, Feb 8, 2026 at 5:38 AM David Hildenbrand (Arm) <david@...nel.org> wrote:
>>
>> On 2/7/26 14:51, Lance Yang wrote:
>>>
>>>
>>>
>>> Good catch!
>>>
>>
>> Good point!
>>
>>>
>>> Right. When the VMA has VM_DROPPABLE, we would drop the lazyfree folio
>>> regardless of whether it (or the PTE) is dirty in try_to_unmap_one().
>>>
>>> So, IMHO, we could go with:
>>>
>>> cc->is_khugepaged && folio_test_lazyfree(folio) &&
>>>       (!pte_dirty(pteval) || (vma->vm_flags & VM_DROPPABLE))
>>
>> Hm. In a VM_DROPPABLE mapping all folios should be marked as lazy-free
>> (see folio_add_new_anon_rmap()).
>>
>> The new (collapse) folio will also be marked lazy (due to
>> folio_add_new_anon_rmap()) free and can just get dropped any time.
>>
>> So likely we should just not skip collapse for lazyfree folios in
>> VM_DROPPABLE mappings?
> 
> 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.

-- 
Cheers,

David