Messages in this thread Patch in this message |  | | From | Waiman Long <> | | Subject | [PATCH] sched/isolation: Don't free memblock allocated cpumasks | | Date | Tue, 5 May 2026 01:18:21 -0400 |
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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()); } } -- 2.53.0
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