Re: [PATCH] sched/isolation: Don't free memblock allocated cpumasks

[Mike Rapoport]

On Mon, May 11, 2026 at 12:55:39AM -0400, Waiman Long wrote:
> On 5/10/26 11:02 AM, Mike Rapoport wrote:
> > Hi Waiman,
> > 
> > On Tue, May 05, 2026 at 01:18:21AM -0400, Waiman Long wrote:
> > > When testing a v7.1 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.
> > > 
> > > [    0.080911] Cannot free reserved memory because of deferred initialization of the memory map
> > > [    0.080911] WARNING: mm/memblock.c:904 at __free_reserved_area+0xde/0xf0, CPU#0: swapper/0/0
> > >    :
> > > [    0.080945] Call Trace:
> > > [    0.080947]  <TASK>
> > > [    0.080949]  memblock_phys_free+0xcb/0x100
> > > [    0.080953]  housekeeping_init+0x14c/0x170
> > > [    0.080957]  start_kernel+0x207/0x450
> > > [    0.080961]  x86_64_start_reservations+0x24/0x30
> > > [    0.080965]  x86_64_start_kernel+0xda/0xe0
> > > [    0.080967]  common_startup_64+0x13e/0x141
> > > [    0.080972]  </TASK>
> > > 
> > > The commit states that freeing of reserved memory before the memory
> > > map is fully initialized in deferred_init_memmap() would cause access
> > > to uninitialized struct pages and may crash when accessing spurious
> > > list pointers. However, if the memblock_free() call is deferred to
> > > the start of initcall processing in the bootup process, for instance,
> > > the following KASAN warning can appear.
> > > 
> > > [    8.514775] BUG: KASAN: use-after-free in memblock_isolate_range+0x4ac/0x650
> > > [    8.514775] Read of size 8 at addr ffff88a07fe6a000 by task swapper/0/1
> > >    :
> > > [    8.514775] Call Trace:
> > > [    8.514775]  <TASK>
> > > [    8.514775]  kasan_report+0xb2/0x1b0
> > > [    8.514775]  memblock_isolate_range+0x4ac/0x650
> > > [    8.514775]  memblock_phys_free+0xc4/0x190
> > > [    8.514775]  housekeeping_late_init+0x257/0x280
> > > [    8.514775]  do_one_initcall+0xaa/0x470
> > > [    8.514775]  do_initcalls+0x1b4/0x1f0
> > > [    8.514775]  kernel_init_freeable+0x4b5/0x550
> > > [    8.514775]  kernel_init+0x1c/0x150
> > > [    8.514775]  ret_from_fork+0x5dc/0x8e0
> > > [    8.514775]  ret_from_fork_asm+0x1a/0x30
> > > [    8.514775]  </TASK>
> > > 
> > > It is likely that memblock_discard() may discard memblock data needed
> > > for memblock_free(). One workaround for now to avoid these warning/bug
> > > messages is to keep the memblock allocated cpumasks even if they are
> > > no longer needed until the memblock subsystem is properly updated to
> > > handle memblock_free().
> > > 
> > > On most systems, memory occuipied by a cpumask is pretty small. So not
> > > much memory will be wasted if the memblock cpumasks are not freed.
> > > 
> > > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > > ---
> > >   kernel/sched/isolation.c | 8 +++++++-
> > >   1 file changed, 7 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> > > index ef152d401fe2..ad9b1a1104e3 100644
> > > --- a/kernel/sched/isolation.c
> > > +++ b/kernel/sched/isolation.c
> > > @@ -189,7 +189,13 @@ 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());
> > > +
> > > +		/*
> > > +		 * TODO: Don't free memblock allocated cpumasks until the
> > > +		 * memblock subystem is able to handle the memblock_free()
> > > +		 * properly.
> > > +		 */
> > > +		// memblock_free(omask, cpumask_size());
> > Before 59bd1d914bb5 it was a silent leak. housekeeping_init() is called
> > after memblock moves all the memory to buddy, so this would only update
> > memblock.reserved.
> > 
> > The comment a few lines above says that we reallocate to be able to kfree()
> > later. Is it possible to delay reallocation until an initcall?
> 
> My original thought was to defer the freeing to init call. That changes led
> to the KASAN bug splat listed in the commit log, I think the right window to
> free memblock memory is currently just too narrow. Do you mean that with the
> fix patch you sent to Breno, memblock freeing in initcall will work without
> bug report?

Yes, with the fix I sent to Breno memblock_free() should work in an
initcall and "do the right thing".

> If so, I can send another patch to defer memblock freeing after the fix
> patch is merged as the KASAN bug is more serious than the memblock
> warning. I will do some testing tomorrow with your fix patch.
 
> Cheers,
> Longman
> 

-- 
Sincerely yours,
Mike.