Re: [PATCH] mm: don't defer freeing kernel page tables while booting

[Lorenzo Stoakes (ARM)]

On Thu, Sep 24, 2026 at 09:59:36AM +0100, Lorenzo Stoakes (ARM) wrote:
> On Thu, Sep 24, 2026 at 09:57:57AM +0100, Lorenzo Stoakes (ARM) wrote:
> > On Thu, Sep 24, 2026 at 09:31:40AM +0200, David Hildenbrand (Arm) wrote:
> > > On 9/24/26 09:28, Mikhail Gavrilov wrote:
> > > > On 9/24/26 09:07, David Hildenbrand (Arm) wrote:
> > > >> Should we instead simply skip the
> > > >>
> > > >> 	schedule_work(&kernel_pgtable_work.work);
> > > >>
> > > >> and rely on anybody freeing stuff later to just free that one alongside?
> > > >>
> > > >> That avoids throwing in more freeing handling.
> > > >
> > > > Yes, that is simpler, and the early table then goes through the same
> > > > IOMMU flush as every other one, so there is no need to reason about
> > > > what an IOMMU can see during boot.  The only cost is that it waits on
> > > > the list until the next kernel page table is freed after boot.
> > >
> > > If we're worried about that actually causing problems we could drain the list at
> > > a later part during the boot stage. I'd suspect we free something else later
> > > already and simply drain the list ...
>
> And on that, yes, I don't think there's anything to worry about there.

Though maybe it's worth ensuring the drain? Shouldn't be hard, so like:

diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index f3754cefb19e..67f286169632 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -457,12 +457,29 @@ static void kernel_pgtable_work_func(struct work_struct *work)
 		__pagetable_free(pt);
 }

+static void schedule_kernel_pgtable_free(void)
+{
+	schedule_work(&kernel_pgtable_work.work);
+}
+
 void pagetable_free_kernel(struct ptdesc *pt)
 {
 	spin_lock(&kernel_pgtable_work.lock);
 	list_add(&pt->pt_list, &kernel_pgtable_work.list);
 	spin_unlock(&kernel_pgtable_work.lock);

-	schedule_work(&kernel_pgtable_work.work);
+	/* No workqueues exist yet. */
+	if (system_state != SYSTEM_BOOTING)
+		schedule_kernel_pgtable_free();
 }
+
+static int kernel_pgtable_drain_early(void)
+{
+	/* Drain any early kernel page table frees. */
+	schedule_kernel_pgtable_free();
+	return 0;
+}
+
+core_initcall(kernel_pgtable_drain_early);
+
 #endif
--
Cheers, Lorenzo