Re: 6.9/BUG: Bad page state in process kswapd0 pfn:d6e840

[David Hildenbrand <david@redhat.com>]

Am 28.05.24 um 15:57 schrieb David Hildenbrand:
> Am 28.05.24 um 08:05 schrieb Mikhail Gavrilov:
>> On Thu, May 23, 2024 at 12:05 PM Mikhail Gavrilov
>> <mikhail.v.gavrilov@gmail.com> wrote:
>>>
>>> On Thu, May 9, 2024 at 10:50 PM David Hildenbrand <david@redhat.com> wrote:
>>>
>>> The only known workload that causes this is updating a large
>>> container. Unfortunately, not every container update reproduces the
>>> problem.
>>
>> Is it possible to add more debugging information to make it clearer
>> what's going on?
> 
> If we knew who originally allocated that problematic page, that might help. 
> Maybe page_owner could give some hints?
> 
>>
>> BUG: Bad page state in process kcompactd0  pfn:605811
>> page: refcount:0 mapcount:0 mapping:0000000082d91e3e index:0x1045efc4f
>> pfn:0x605811
>> aops:btree_aops ino:1
>> flags: 
>> 0x17ffffc600020c(referenced|uptodate|workingset|node=0|zone=2|lastcpupid=0x1fffff)
>> raw: 0017ffffc600020c dead000000000100 dead000000000122 ffff888159075220
>> raw: 00000001045efc4f 0000000000000000 00000000ffffffff 0000000000000000
>> page dumped because: non-NULL mapping
> 
> Seems to be an order-0 page, otherwise we would have another "head: ..." report.
> 
> It's not an anon/ksm/non-lru migration folio, because we clear the page->mapping 
> field for them manually on the page freeing path. Likely it's a pagecache folio.
> 
> So one option is that something seems to not properly set folio->mapping to 
> NULL. But that problem would then also show up without page migration? Hmm.
> 
>> Hardware name: ASUS System Product Name/ROG STRIX B650E-I GAMING WIFI,
>> BIOS 2611 04/07/2024
>> Call Trace:
>>   <TASK>
>>   dump_stack_lvl+0x84/0xd0
>>   bad_page.cold+0xbe/0xe0
>>   ? __pfx_bad_page+0x10/0x10
>>   ? page_bad_reason+0x9d/0x1f0
>>   free_unref_page+0x838/0x10e0
>>   __folio_put+0x1ba/0x2b0
>>   ? __pfx___folio_put+0x10/0x10
>>   ? __pfx___might_resched+0x10/0x10
> 
> I suspect we come via
>      migrate_pages_batch()->migrate_folio_unmap()->migrate_folio_done().
> 
> Maybe this is the "Folio was freed from under us. So we are done." path
> when "folio_ref_count(src) == 1".
> 
> Alternatively, we might come via
>      migrate_pages_batch()->migrate_folio_move()->migrate_folio_done().
> 
> For ordinary migration, move_to_new_folio() will clear src->mapping if
> the folio was migrated successfully. That's the very first thing that 
> migrate_folio_move() does, so I doubt that is the problem.
> 
> So I suspect we are in the migrate_folio_unmap() path. But for
> a !anon folio, who should be freeing the folio concurrently (and not clearing 
> folio->mapping?)? After all, we have to hold the folio lock while migrating.
> 
> In khugepaged:collapse_file() we manually set folio->mapping = NULL, before 
> dropping the reference.
> 
> Something to try might be (to see if the problem goes away).
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index dd04f578c19c..45e92e14c904 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1124,6 +1124,13 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>                  /* Folio was freed from under us. So we are done. */
>                  folio_clear_active(src);
>                  folio_clear_unevictable(src);
> +               /*
> +                * Anonymous and movable src->mapping will be cleared by
> +                * free_pages_prepare so don't reset it here for keeping
> +                * the type to work PageAnon, for example.
> +                */
> +               if (!folio_mapping_flags(src))
> +                       src->mapping = NULL;
>                  /* free_pages_prepare() will clear PG_isolated. */
>                  list_del(&src->lru);
>                  migrate_folio_done(src, reason);
> 
> But it does feel weird: who freed the page concurrently and didn't clear 
> folio->mapping ...
> 
> We don't hold the folio lock of src, though, but have the only reference. So
> another possible thing might be folio refcount mis-counting: folio_ref_count() 
> == 1 but there are other references (e.g., from the pagecache).

Hmm, your original report mentions kswapd, so I'm getting the feeling someone 
does one folio_put() too much and we are freeing a pageache folio that is still 
in the pageache and, therefore, has folio->mapping set ... bisecting would 
really help.

-- 
Thanks,

David / dhildenb