Messages in this thread |  | | Date | Mon, 11 May 2026 17:36:08 -0400 | | Subject | Re: [PATCH] sched/isolation: Don't free memblock allocated cpumasks | | From | Waiman Long <> |
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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>
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