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

[David Hildenbrand <david@redhat.com>]

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 :)

-- 
Cheers,

David / dhildenb