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

[Frederic Weisbecker <frederic@kernel.org>]

Le Wed, Jul 01, 2026 at 10:25:59AM -0400, Phil Auld a écrit :
> 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@redhat.com/
> > > 
> > > Fixes: 27c3a5967f05 ("sched/isolation: Convert housekeeping cpumasks to rcu pointers")
> > > Signed-off-by: Waiman Long <longman@redhat.com>
> > > ---
> > >  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 :)

It tells the truth, just a bit too much :-)

-- 
Frederic Weisbecker
SUSE Labs