Re: [PATCH v2 4/4] mm/mlock: migrate folios out of CMA when mlocking a range

[Wandun <chenwandun1@gmail.com>]

On 7/9/26 17:13, Sebastian Andrzej Siewior wrote:
> On 2026-07-07 15:54:57 [+0100], Lorenzo Stoakes wrote:
>> On Tue, Jul 07, 2026 at 08:59:25PM +0800, Wandun Chen wrote:
>>> From: Wandun Chen <chenwandun@lixiang.com>
>>>
>>> The region covered by mlock[all] may contain CMA pages. cma_alloc installs
>>> migration entries in the page table, if a memory access occurs at this
>>> point, it must wait for the migration to complete, which may cause
>>> latency spikes on the RT kernels.
>>>
>>> Try to move the migration cost into the mlock[all] caller, which is
>>> typically a setup path. So reduce the chance of latency spikes on RT
>>> kernels by migrating the currently mapped CMA pages out of CMA region.
>>
>> 'reduce the chances of latency' so do you have any data to back this invasive
>> change or not?
> 
> The application gets pages assigned which belong a CMA area. If the
> pages are in need by the CMA then those pages are replaced with other
> pages during a migration phase. Since there is no guarantee how long
> this will take and is also subject to general scheduling in the system
> it will be measurable and painful once hit.
> 
>> And for RT, but nothing in here at all checks for RT? You're using this
>> compaction sysctl as an RT check somehow? That's gross.
> 
> The man-page for mlock says that it guarantees to stay in RAM and/ or
> preventing to be moved to swap area. I would however argue that
> replacing physical pages in the background should fall under this since
> the application can't access them and is blocked while trying. The notes
> section (in the man-page) lists "real-time applications" and
> "deterministic timing".  Therefore I think it makes sense to do this
> unconditionally for mlock areas regardless of the sysctl knob.
> Security related application probably only care that their memory does
> not hit the swap area and probably wouldn't mind 10ms delay.

Thanks for helping to explain, that helps a lot, Sebastian.
Wandun
> 
>> This doesn't feel like the right solution.
> 
> Sebastian