Re: [PATCH mm-unstable v1 2/3] mm/migrate.c: Prevent folio splitting from interacting with KSM

[David Hildenbrand (Arm)]

On 7/15/26 06:05, Nico Pache wrote:
> On Tue, Jun 9, 2026 at 11:27 AM Usama Arif <usama.arif@xxxxxxxxx> wrote:
>>
>> On Tue, 09 Jun 2026 10:07:20 -0400 Zi Yan <ziy@xxxxxxxxxx> wrote:
>>
>>>
> 
> This would be a good intermediate solution while allowing backward
> compatibility.

And an absolute horrible user-visible toggle :)

> 
>>>
>>> Just think out loud. Or just skip huge pages all the time unless memory pressure
>>> is present. Basically treat KSM as a way of reducing memory pressure by merging
>>> pages.
> 
> Hmm interesting. We should decide on future changes before committing
> to one direction; however, i think the immediate solution would be
> what I currently have prepped for v2.
> 
> My v2 follows my RFC alternative approach: skip zeropage remapping if
> the split comes from KSM. It also contains the patch (3) from this
> version, which skips zeropage remapping if the shrinker is disabled.
> 
>>
>> I do agree with this. I questioned about if it even makes sense to split THP
>> with KSM in RFC (https://lore.kernel.org/all/20260510114001.600681-1-usama.arif@xxxxxxxxx/)
> 
> This is indeed a good question but I don't know the right solution.
> 
> Perhaps we can discuss it here.
> 
> So far here are the potential solutions:
> a) skip zeropage remapping if the split comes from KSM
> b) skip zeropage remapping completely if the shrinker is disabled
> c) Disable KSM THP splitting completely
> d) add a skip_huge_page sysfs toggle
> e) only scan THPs when memory pressure is present
> 
> (a)+(b) would be my V2 which I can send out whenever.
> 
> (c) is the nuclear option, but perhaps KSM for THP is rather
> inefficient and pointless
> 
> (d) could be used with (a) and (b) and allows more flexibility than (c)
> 
> (e) is a more dynamic approach, and could be combined with (b) and (d)
> 
> Let me know what you think :)

Where does the current patch set fall into and what's the problem with it?

-- 
Cheers,

David