Re: [DISCUSSION] Unconditionally lock folios when calling rmap_walk()

[David Hildenbrand <david@redhat.com>]

On 02.09.25 20:59, Lokesh Gidra wrote:
> On Fri, Aug 29, 2025 at 2:04 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.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