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

[Waiman Long]

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());
 	}
 }
 
-- 
2.53.0