Re: [PATCH v3.1] sched/isolation: Defer freeing of cpumask memblock memory to initcall

[Phil Auld]

On Wed, Jul 01, 2026 at 10:13:57AM -0400 Phil Auld wrote:
> Hi Waiman,
> 
> On Thu, Jun 04, 2026 at 02:24:40PM -0400 Waiman Long wrote:
> > When testing a linux-next kernel with commit 59bd1d914bb5 ("memblock:
> > warn when freeing reserved memory before memory map is initialized"),
> > the following warning was hit when there was a "nohz_full" kernel boot
> > parameter.
> > 
> >   Cannot free reserved memory because of deferred initialization of the memory map
> >   WARNING: mm/memblock.c:904 at __free_reserved_area+0xde/0xf0, CPU#0: swapper/0/0
> >     :
> >   Call Trace:
> >    <TASK>
> >    memblock_phys_free+0xcb/0x100
> >    housekeeping_init+0x14c/0x170
> >    start_kernel+0x207/0x450
> >    x86_64_start_reservations+0x24/0x30
> >    x86_64_start_kernel+0xda/0xe0
> >    common_startup_64+0x13e/0x141
> >    </TASK>
> > 
> > IOW, we shouldn't free memblock allocated memory so early
> > in the boot process when memory map isn't fully initialized in
> > deferred_init_memmap().
> > 
> > Fix it by saving the housekeeping cpumask memblock memory to
> > be freed into a free list in housekeeping_init() and add a new
> > housekeeping_late_init() helper to defer the actual freeing of memblock
> > memory to when initcall's are being processed. The non-atomic version
> > of the llist APIs are used as there is no contention.
> > 
> > This commit also depends on the presence of commit 7c2eee9c1367
> > ("memblock: don't touch memblock arrays when memblock_free() is called
> > late") to prevent a KASAN UAF bug report [1].
> > 
> >  [1] https://lore.kernel.org/lkml/20260505051821.1107133-1-longman@xxxxxxxxxx/
> > 
> > Fixes: 27c3a5967f05 ("sched/isolation: Convert housekeeping cpumasks to rcu pointers")
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >  kernel/sched/isolation.c | 16 +++++++++++++++-
> >  1 file changed, 15 insertions(+), 1 deletion(-)
> > 
> >  [v3.1] Add __initdata to memblock_freelist
> > 
> > diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> > index ef152d401fe2..156025ef81b7 100644
> > --- a/kernel/sched/isolation.c
> > +++ b/kernel/sched/isolation.c
> > @@ -8,6 +8,7 @@
> >   *
> >   */
> >  #include <linux/sched/isolation.h>
> > +#include <linux/llist.h>
> >  #include <linux/pci.h>
> >  #include "sched.h"
> >  
> > @@ -27,6 +28,7 @@ struct housekeeping {
> >  };
> >  
> >  static struct housekeeping housekeeping;
> > +static __initdata LLIST_HEAD(memblock_freelist);
> >  
> >  bool housekeeping_enabled(enum hk_type type)
> >  {
> > @@ -189,10 +191,22 @@ void __init housekeeping_init(void)
> >  		WARN_ON_ONCE(cpumask_empty(omask));
> >  		cpumask_copy(nmask, omask);
> >  		RCU_INIT_POINTER(housekeeping.cpumasks[type], nmask);
> > -		memblock_free(omask, cpumask_size());
> > +		__llist_add((struct llist_node *)omask, &memblock_freelist);
> 
> This cast is somewhat concerning. I think I see why it's needed. Wrapping
> it in a proper struct would require more allocating and freeing and
> make the problem worse. It should work though.
> 
>

Fwiw, opencode/sonnet suggested a comment like this:

/*
 * We can't allocate wrapper structs from memblock as they'd need
 * deferred freeing too. Instead, reuse the cpumask memory itself
 * as llist nodes. This is safe because:
 * - cpumask_size() >= sizeof(struct llist_node)
 * - Memory is properly aligned (SMP_CACHE_BYTES)
 * - The cpumask is never accessed after being added to the list
 */          

... which may be overkill :)



Cheers,
Phil


> Reviewed-by: Phil Auld <pauld@xxxxxxxx>
> 
> 
> 
> 
> Cheers,
> Phil
> 
> 
> >  	}
> >  }
> >  
> > +static int __init housekeeping_late_init(void)
> > +{
> > +	struct llist_node *llnode, *pos, *t;
> > +
> > +	/* Free allocated memblock memory, if any */
> > +	llnode = __llist_del_all(&memblock_freelist);
> > +	llist_for_each_safe(pos, t, llnode)
> > +		memblock_free(pos, cpumask_size());
> > +	return 0;
> > +}
> > +pure_initcall(housekeeping_late_init);
> > +
> >  static void __init housekeeping_setup_type(enum hk_type type,
> >  					   cpumask_var_t housekeeping_staging)
> >  {
> > -- 
> > 2.54.0
> > 
> > 
> 
> -- 
> 
> 

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