Re: [PATCH 2/2] hwpoison, memory_hotplug: lock folio before unmap hwpoisoned folio

[mawupeng <mawupeng1@huawei.com>]

On 2025/1/13 20:35, David Hildenbrand wrote:
> On 13.01.25 09:27, Wupeng Ma wrote:
>> From: Ma Wupeng <mawupeng1@huawei.com>
>>
>> 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@huawei.com>
>> ---
>>   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.

>