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

[Waiman Long <longman@redhat.com>]

On 5/6/26 9:25 AM, Valentin Schneider wrote:
> On 05/05/26 01:18, 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>
>>
> Darn, I just saw the previous version doing this.
>
>> 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().
> Pardon my ignorance, but how come this isn't the case for the other
> memblock users? It sounds like there is no right place for freeing this
> mask.

My current thought is to chain all the memblock memory blocks to be 
freed in a singly linked list first and then freed them at the right 
moment by the memblock code. That will require some more investigation 
into the memblock code. This patch is just a temporary workaround which 
I hope will be reverted in the future.

Cheers,
Longman