Re: [PATCH 2/2] hwpoison, memory_hotplug: lock folio before unmap hwpoisoned folio
From: David Hildenbrand
Date: Tue Jan 14 2025 - 04:40:40 EST
On 14.01.25 10:08, mawupeng wrote:
On 2025/1/13 20:35, David Hildenbrand wrote:
On 13.01.25 09:27, Wupeng Ma wrote:
From: Ma Wupeng <mawupeng1@xxxxxxxxxx>
Commit b15c87263a69 ("hwpoison, memory_hotplug: allow hwpoisoned pages to
be offlined) add page poison checks in do_migrate_range in order to make
offline hwpoisoned page possible by introducing isolate_lru_page and
try_to_unmap for hwpoisoned page. However folio lock must be held before
calling try_to_unmap. Add it to fix this problem.
Waring will be produced if filio is not locked during unmap:
------------[ cut here ]------------
kernel BUG at ./include/linux/swapops.h:400!
Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
Modules linked in:
CPU: 4 UID: 0 PID: 411 Comm: bash Tainted: G W 6.13.0-rc1-00016-g3c434c7ee82a-dirty #41
Tainted: [W]=WARN
Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015
pstate: 40400005 (nZcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : try_to_unmap_one+0xb08/0xd3c
lr : try_to_unmap_one+0x3dc/0xd3c
Call trace:
try_to_unmap_one+0xb08/0xd3c (P)
try_to_unmap_one+0x3dc/0xd3c (L)
rmap_walk_anon+0xdc/0x1f8
rmap_walk+0x3c/0x58
try_to_unmap+0x88/0x90
unmap_poisoned_folio+0x30/0xa8
do_migrate_range+0x4a0/0x568
offline_pages+0x5a4/0x670
memory_block_action+0x17c/0x374
memory_subsys_offline+0x3c/0x78
device_offline+0xa4/0xd0
state_store+0x8c/0xf0
dev_attr_store+0x18/0x2c
sysfs_kf_write+0x44/0x54
kernfs_fop_write_iter+0x118/0x1a8
vfs_write+0x3a8/0x4bc
ksys_write+0x6c/0xf8
__arm64_sys_write+0x1c/0x28
invoke_syscall+0x44/0x100
el0_svc_common.constprop.0+0x40/0xe0
do_el0_svc+0x1c/0x28
el0_svc+0x30/0xd0
el0t_64_sync_handler+0xc8/0xcc
el0t_64_sync+0x198/0x19c
Code: f9407be0 b5fff320 d4210000 17ffff97 (d4210000)
---[ end trace 0000000000000000 ]---
Fixes: b15c87263a69 ("hwpoison, memory_hotplug: allow hwpoisoned pages to be offlined")
Signed-off-by: Ma Wupeng <mawupeng1@xxxxxxxxxx>
---
mm/memory_hotplug.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 330668d37e44..9bedecfc3577 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1805,8 +1805,12 @@ static void do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
(folio_test_large(folio) && folio_test_has_hwpoisoned(folio))) {
if (WARN_ON(folio_test_lru(folio)))
folio_isolate_lru(folio);
- if (folio_mapped(folio))
+ if (folio_mapped(folio)) {
+ folio_lock(folio);
unmap_poisoned_folio(folio, TTU_IGNORE_MLOCK | TTU_HWPOISON);
+ folio_unlock(folio);
+ }
The comment above says "have elevated reference counts", but I I wonder if this code could race with un-poisoning (although probably a rare event).
If there is an elevated reference already, why not move that chunk after the folio_try_get() and just drop the comment that describes the implied magic?
I mean, migration of hwpoison is dangerous either way, so that's not the biggest problem I guess :)
AFAICT during memory_failure, the refcount will not be cleared for poisoned page in order to keep it away from free buddy pages.
So move that chunk after the folio_try_get() do seems nice and poisoned free pages do seems weird to me.
If the "free" poisoned pages have a raised refcount, folio_try_get()
would be able to grab them to essentially skip them (not mapped).
If the "free" poisoned pages don't have a raised refcount (impossible
right now IIRC), folio_try_get() would skip, them, which would be the
right thing to do.
So I think that should work.
--
Cheers,
David / dhildenb