Re: [PATCH v2 2/4] mm/mlock: wait for migration to finish when mlocking a folio

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-07-07 15:33:32 [+0100], Lorenzo Stoakes wrote:
> On Tue, Jul 07, 2026 at 08:59:23PM +0800, Wandun Chen wrote:
> > From: Wandun Chen <chenwandun@lixiang.com>
> >
> > In RT kernels, sysctl_compact_unevictable_allowed is false by default,
> > when the mlock/mlockall system call try to lock all the present page,
> > the mlock_pte_range function skips non-present entries. If these
> > non-present entries are migration entries, and the migration is not
> > guaranteed to have completed before the mlock/mlockall, it may result
> > in a page fault on subsequent access, which then waits for the
> > migration to finish, causing spike latency in RT kernels.
> 
> Is this really noticable and measurable?

You want to avoid page faults like this which is why you do mlock in the
first place.
If the thread has a RT priority and it blocks here then other RT threads
will be scheduled and your migration thread, which will resolve this, is
SCHED_OTHER and the last one in the line. Assuming that thread has a 1ms
cycle/ deadline then it will likely miss it.

> I suppose not too many ranges should be mock

Sebastian