linux-kernel - Re: [DISCUSSION] Unconditionally lock folios when calling rmap

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: <d61b173f-ed58-4d60-b1a1-b93678371576@redhat.com>
Date: Tue, 2 Sep 2025 21:01:38 +0200
From: David Hildenbrand <david@...hat.com>
To: Lokesh Gidra <lokeshgidra@...gle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Harry Yoo <harry.yoo@...cle.com>, Zi Yan <ziy@...dia.com>,
 Barry Song <21cnbao@...il.com>,
 "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
 Peter Xu <peterx@...hat.com>, Suren Baghdasaryan <surenb@...gle.com>,
 Kalesh Singh <kaleshsingh@...gle.com>, android-mm <android-mm@...gle.com>,
 linux-kernel <linux-kernel@...r.kernel.org>, Jann Horn <jannh@...gle.com>,
 Rik van Riel <riel@...riel.com>, Vlastimil Babka <vbabka@...e.cz>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>
Subject: Re: [DISCUSSION] Unconditionally lock folios when calling rmap_walk()

On 02.09.25 20:59, Lokesh Gidra wrote:
> On Fri, Aug 29, 2025 at 2:04 AM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.com> wrote:
>>
>> On Fri, Aug 29, 2025 at 10:42:45AM +0200, David Hildenbrand wrote:
>>>>>
>>>>> I do wonder if we can identify this case and handle things differently.
>>>>>
>>>>> Perhaps even saying 'try and get the rmap lock, but if there's "too much"
>>>>> contention, grab the folio lock.
>>>>
>>>> Can you please elaborate what you mean? Where do you mean we can
>>>> possibly do something like this?
>>>>
>>>> UFFD move only works on PageAnonExclusive folios. So, would it help
>>>> (in terms of avoiding contention) if we were to change the condition:
>>>
>>> I think we shouldn't be using PAE here. Once could consider using
>>> folio_maybe_mapped_shared(), and assume contention on the folio lock if it
>>> is maybe mapped shared.
>>
>> Interesting!
>>
>>>
>>> But the real question is with whom we would be contending for the folio
>>> lock.
>>>
>>> Is it really other processes mapping that folio? I'm not so sure.
>>
>> Yeah, I might go off and do some research myself on this, actually. Nail down
>> wehre this might actually happen.
>>
>> Generally I'm softening on this and maybe we're good with the proposed change.
>>
>> But still want to be super careful here... :)
>>
> Anxiously waiting for your assessment. Fingers crossed :)

I'd suggest you prepare an RFC patch where you neatly summarize all we 
learned so far. :)

-- 
Cheers

David / dhildenb