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

[Waiman Long <longman@redhat.com>]

On 5/11/26 4:34 AM, Mike Rapoport wrote:
> 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@redhat.com>
>>>> ---
>>>>    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".

Thanks for the confirmation. I have tested your patch with my patch to 
defer the memblock_free() to initcall. There is no longer any KASAN 
splat when booting up a debug test kernel. You can add the following tag 
when you send out your patch.

Tested-by: Waiman Long <longman@redhat.com>