Re: linux-next regression: SNP Guest boot hangs with certain cpu/mem config combination

[Kirill A. Shutemov]

On Thu, Mar 27, 2025 at 07:39:22PM +0200, Kirill A. Shutemov wrote:
> On Thu, Mar 27, 2025 at 11:02:24AM -0400, Steven Rostedt wrote:
> > On Thu, 27 Mar 2025 16:43:43 +0200
> > "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > 
> > > > > The only option I see so far is to drop static branch from this path.
> > > > > 
> > > > > But I am not sure if it the only case were we use static branch from CPU
> > > > > hotplug callbacks.
> > > > > 
> > > > > Any other ideas?  
> > > > 
> > > > 
> > > > Hmmm, didn't take too close a look here, but there is the
> > > > static_key_slow_dec_cpuslocked() variant, would that work here? Is the issue
> > > > the caller may or may not have the cpu_hotplug lock?  
> > > 
> > > Yes. This is generic page alloc path and it can be called with and without
> > > the lock.
> > 
> > Note, it's not the static_branch that is an issue, it's enabling/disabling
> > the static branch that is. Changing a static branch takes a bit of work as
> > it does modify the kernel text.
> > 
> > Is it possible to delay the update via a workqueue?
> 
> Ah. Good point. Should work. I'll give it try.

The patch below fixes problem for me.

It is silly to add work_struct to zone for one-time cleanup, but I guess
not a big deal.

Tom, Srikanth, could you please test it?

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 9540b41894da..ea5f7e0b675d 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -964,6 +964,9 @@ struct zone {
 #ifdef CONFIG_UNACCEPTED_MEMORY
 	/* Pages to be accepted. All pages on the list are MAX_PAGE_ORDER */
 	struct list_head	unaccepted_pages;
+
+	/* To be called once last page in the zone is accepted */
+	struct work_struct	unaccepted_cleanup;
 #endif
 
 	/* zone flags, see below */
diff --git a/mm/internal.h b/mm/internal.h
index 109ef30fee11..f2e6d42af6eb 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1516,6 +1516,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 
 #ifdef CONFIG_UNACCEPTED_MEMORY
 void accept_page(struct page *page);
+void unaccepted_cleanup_work(struct work_struct *work);
 #else /* CONFIG_UNACCEPTED_MEMORY */
 static inline void accept_page(struct page *page)
 {
diff --git a/mm/mm_init.c b/mm/mm_init.c
index 2630cc30147e..d5a51f65dc4d 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1404,6 +1404,7 @@ static void __meminit zone_init_free_lists(struct zone *zone)
 
 #ifdef CONFIG_UNACCEPTED_MEMORY
 	INIT_LIST_HEAD(&zone->unaccepted_pages);
+	INIT_WORK(&zone->unaccepted_cleanup, unaccepted_cleanup_work);
 #endif
 }
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4fe93029bcb6..e5f16fdb7eeb 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6921,6 +6921,11 @@ static DEFINE_STATIC_KEY_FALSE(zones_with_unaccepted_pages);
 
 static bool lazy_accept = true;
 
+void unaccepted_cleanup_work(struct work_struct *work)
+{
+	static_branch_dec(&zones_with_unaccepted_pages);
+}
+
 static int __init accept_memory_parse(char *p)
 {
 	if (!strcmp(p, "lazy")) {
@@ -6960,7 +6965,7 @@ static void __accept_page(struct zone *zone, unsigned long *flags,
 	__free_pages_ok(page, MAX_PAGE_ORDER, FPI_TO_TAIL);
 
 	if (last)
-		static_branch_dec(&zones_with_unaccepted_pages);
+		schedule_work(&zone->unaccepted_cleanup);
 }
 
 void accept_page(struct page *page)
-- 
  Kiryl Shutsemau / Kirill A. Shutemov