Re: [PATCH v1] mm: recheck PageCompound() before printing out info of head page

[Miaohe Lin]

On 2022/4/15 9:08, John Hubbard wrote:
> On 4/14/22 16:59, Naoya Horiguchi wrote:
>> From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
>>
>> The following VM_BUG_ON_FOLIO() is triggered when memory error event
>> happens on the (thp/folio) pages which are about to be freed:
>>
>>    [ 1160.232771] page:00000000b36a8a0f refcount:1 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x16a000
>>    [ 1160.236916] page:00000000b36a8a0f refcount:0 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x16a000
>>    [ 1160.240684] flags: 0x57ffffc0800000(hwpoison|node=1|zone=2|lastcpupid=0x1fffff)
>>    [ 1160.243458] raw: 0057ffffc0800000 dead000000000100 dead000000000122 0000000000000000
>>    [ 1160.246268] raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000
>>    [ 1160.249197] page dumped because: VM_BUG_ON_FOLIO(!folio_test_large(folio))
>>    [ 1160.251815] ------------[ cut here ]------------
>>    [ 1160.253438] kernel BUG at include/linux/mm.h:788!
>>    [ 1160.256162] invalid opcode: 0000 [#1] PREEMPT SMP PTI
>>    [ 1160.258172] CPU: 2 PID: 115368 Comm: mceinj.sh Tainted: G            E     5.18.0-rc1-v5.18-rc1-220404-2353-005-g83111+ #3
>>    [ 1160.262049] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1.fc35 04/01/2014
>>    [ 1160.265103] RIP: 0010:dump_page.cold+0x27e/0x2bd
>>    [ 1160.266757] Code: fe ff ff 48 c7 c6 81 f1 5a 98 e9 4c fe ff ff 48 c7 c6 a1 95 59 98 e9 40 fe ff ff 48 c7 c6 50 bf 5a 98 48 89 ef e8 9d 04 6d ff <0f> 0b 41 f7 c4 ff 0f 00 00 0f 85 9f fd ff ff 49 8b 04 24 a9 00 00
>>    [ 1160.273180] RSP: 0018:ffffaa2c4d59fd18 EFLAGS: 00010292
>>    [ 1160.274969] RAX: 000000000000003e RBX: 0000000000000001 RCX: 0000000000000000
>>    [ 1160.277263] RDX: 0000000000000001 RSI: ffffffff985995a1 RDI: 00000000ffffffff
>>    [ 1160.279571] RBP: ffffdc9c45a80000 R08: 0000000000000000 R09: 00000000ffffdfff
>>    [ 1160.281794] R10: ffffaa2c4d59fb08 R11: ffffffff98940d08 R12: ffffdc9c45a80000
>>    [ 1160.283920] R13: ffffffff985b6f94 R14: 0000000000000000 R15: ffffdc9c45a80000
>>    [ 1160.286641] FS:  00007eff54ce1740(0000) GS:ffff99c67bd00000(0000) knlGS:0000000000000000
>>    [ 1160.289498] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>    [ 1160.291106] CR2: 00005628381a5f68 CR3: 0000000104712003 CR4: 0000000000170ee0
>>    [ 1160.293031] Call Trace:
>>    [ 1160.293724]  <TASK>
>>    [ 1160.294334]  get_hwpoison_page+0x47d/0x570
>>    [ 1160.295474]  memory_failure+0x106/0xaa0
>>    [ 1160.296474]  ? security_capable+0x36/0x50
>>    [ 1160.297524]  hard_offline_page_store+0x43/0x80
>>    [ 1160.298684]  kernfs_fop_write_iter+0x11c/0x1b0
>>    [ 1160.299829]  new_sync_write+0xf9/0x160
>>    [ 1160.300810]  vfs_write+0x209/0x290
>>    [ 1160.301835]  ksys_write+0x4f/0xc0
>>    [ 1160.302718]  do_syscall_64+0x3b/0x90
>>    [ 1160.303664]  entry_SYSCALL_64_after_hwframe+0x44/0xae
>>    [ 1160.304981] RIP: 0033:0x7eff54b018b7
>>
>> As shown in the RIP address, this VM_BUG_ON in folio_entire_mapcount() is
>> called from dump_page("hwpoison: unhandlable page") in get_any_page().
>> The below explains the mechanism of the race:
>>
>>    CPU 0                                       CPU 1
>>
>>      memory_failure
>>        get_hwpoison_page
>>          get_any_page
>>            dump_page
>>              compound = PageCompound
>>                                                  free_pages_prepare
>>                                                    page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP
>>              folio_entire_mapcount
>>                VM_BUG_ON_FOLIO(!folio_test_large(folio))
>>
>> So in order to avoid the BUG_ON, recheck PageCompound() before __dump_page()
>> prints out information about the head page.  Precisely saying, this does
>> not close up the race, but mitigates the impact to just inaccurate info,
>> which should be acceptable.
> 
> Actually, I don't think it really closes the race even to that extent,
> because a machine check (MCE, on x86) could come in at any point, so
> rechecking (without locks) won't really completely fix such a bug, right?

It might also be unsafe to do dump_mapping or dump_page_owner when page could
be freed at any time or we might come across some use-after-free issues?

> 
>>
>> Fixes: 74e8ee4708a8 ("mm: Turn head_compound_mapcount() into folio_entire_mapcount()")
>> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
>> ---
>> I'm actually not sure how widely the race condition could affect.
>> If we can consider this as a bug specific to memory_failure(), simply
>> replacing the dump_page() with simple printk() might be a proper fix
> 
> I think we should have separate, limited, special case functions that
> are safe for use by exotic things like the MCE handler. And so just
> doing a printk there seems better to me.
> 
>> (I confirmed that that fix also works).  But if any other dump_page()'s
>> caller could race with freeing thp/folio, the more generic fix
>> (I'm suggesting in this patch) would be desirable.
> 
> *If* there are other races, they should be addressed separately, IMHO.
> Because MCE should be a special case.
> 
>> ---
>>   mm/debug.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/debug.c b/mm/debug.c
>> index bef329bf28f0..fce63b200bea 100644
>> --- a/mm/debug.c
>> +++ b/mm/debug.c
>> @@ -93,7 +93,7 @@ static void __dump_page(struct page *page)
>>       pr_warn("page:%p refcount:%d mapcount:%d mapping:%p index:%#lx pfn:%#lx\n",
>>               page, page_ref_count(head), mapcount, mapping,
>>               page_to_pgoff(page), page_to_pfn(page));
>> -    if (compound) {
>> +    if (compound && PageCompound(page)) {
> 
> If people decide to keep this approach, then I'd go with something
> that makes it clear, like:
> 
>     /* Re-check, in a semi-doomed attempt to avoid racing with MCE: */
>     compound = PageCompound(page);

Agree. A comment will be needed or the code might look obscure.

Thanks!

> 
> :)
> 
> thanks,