get_pageblock_bitmap() sometimes returns a NULL pointer which needs to be properly handled

Markus Gothe
2024-05-02T16:02:14Z

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Hi,
under some rare occasion I run into the following crash:

[ 41.417606] pstate: 80400005 (Nzcv daif +PAN -UAO)
[ 41.422406] pc : set_pfnblock_flags_mask+0x50/0x94
[ 41.427193] lr : compaction_alloc+0x220/0x804
[ 41.431544] sp : ffffffc01104bb10
[ 41.434852] x29: ffffffc01104bb10 x28: ffffffc010e5b500
[ 41.440165] x27: 0000000000098000 x26: ffffffc010e5b500
[ 41.445477] x25: 0000000000000066 x24: 0000000000090800
[ 41.450789] x23: 0000000000000200 x22: 0000000000084000
[ 41.456093] x21: ffffffc010e82000 x20: ffffffc010b88000
[ 41.461396] x19: ffffffc01104bd70 x18: 0000000000000000
[ 41.466700] x17: f1f24e35df34dda4 x16: 6b3f63a0e1157268
[ 41.472004] x15: 4b3990ec2568ada0 x14: 757ebc126939cb5f
[ 41.477308] x13: 9df9488aba179ccb x12: 0000000000000000
[ 41.482612] x11: 0000000000000000 x10: ffffffc010c5fc30
[ 41.487916] x9 : ffffff801eea7c00 x8 : 000000001bf00000
[ 41.493219] x7 : 0000000000000000 x6 : 000000000000003f
[ 41.498525] x5 : 0000000000000108 x4 : 1000000000000000
[ 41.503835] x3 : 0000000000000021 x2 : 000000000000003c
[ 41.509139] x1 : 0000000000000001 x0 : 0000000000000003
[ 41.514443] Call trace:
[ 41.516887] set_pfnblock_flags_mask+0x50/0x94
[ 41.521330] migrate_pages+0x90/0x7f0
[ 41.524992] compact_zone+0x854/0x9f0
[ 41.528647] kcompactd_do_work+0x168/0x230
[ 41.532734] kcompactd+0x58/0x140
[ 41.536043] kthread+0x120/0x124
[ 41.539263] ret_from_fork+0x10/0x24
[ 41.542835] Code: d346fc43 4b0000c2 8b030ce5 9ac22084 (f86378e0)
[ 41.548925] ---[ end trace 731400a587304db3 ]---

I've pin-pointed it down to pageblock_flags pointer being initialized to NULL under certain conditions. I don't know why this happens.
Maybe it is some obscure race condition which only shows up on my system.

Nonetheless, I've made a fix for this in the attached patch. It adds sanity checking and protects the system for crashing.
I think it is better to be safe than sorry.

--
Best Regards,
Markus

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From 2ff8267436362532848b73b7beab7fd36015b0c5 Mon Sep 17 00:00:00 2001
From: Markus Gothe <markus.gothe@genexis.eu>
Date: Thu, 2 May 2024 15:36:36 +0200
Subject: [PATCH] page_alloc.c: Sanity check for NULL pointers.

get_pageblock_bitmap() might in very rare cicumstances
return NULL which must be handled accordingly or
otherwise we will end up with a kernel crash.
---
mm/page_alloc.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 540af9a120e6..ab230e349862 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -486,6 +486,9 @@ static __always_inline unsigned long __get_pfnblock_flags_mask(struct page *page
unsigned long word;

bitmap = get_pageblock_bitmap(page, pfn);
+ if (unlikely(bitmap == NULL))
+ return 0;
+
bitidx = pfn_to_bitidx(page, pfn);
word_bitidx = bitidx / BITS_PER_LONG;
bitidx &= (BITS_PER_LONG-1);
@@ -528,6 +531,9 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits));

bitmap = get_pageblock_bitmap(page, pfn);
+ if (unlikely(bitmap == NULL))
+ return;
+
bitidx = pfn_to_bitidx(page, pfn);
word_bitidx = bitidx / BITS_PER_LONG;
bitidx &= (BITS_PER_LONG-1);
--
2.43.2