RE: [PATCH 2/2] mm: debug: print correct information for slab folios

[Sukrit.Bhatnagar@xxxxxxxx]

Hi Matthew,

On 2024-05-22 21:32, Matthew Wilcox wrote:
> On Wed, May 22, 2024 at 04:46:29PM +0900, Sukrit Bhatnagar wrote:
>> If the folio tests true for slab, do not print information that does not
>> apply to it. Instead, print the slab flags stored in the kmem_cache field.
>> 
>> [    7.248722] page: refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff888103e6aa87>
>> [    7.249135] head: order:3 entire_mapcount:0 nr_pages_mapped:0 pincount:0
>> [    7.249429] slab flags: 0x8000000000000840(slab|head|zone=2)
>> [    7.249664] cache flags: 0x10310(HWCACHE_ALIGN|PANIC|TYPESAFE_BY_RCU|CMPXCHG_DOUBLE)
>> [    7.249999] raw: 8000000000000000 ffffea00040f9a01 ffffea00040f9bc8 dead000000000400
> 
> You haven't tested this against the current codebase ...
>
>> @@ -98,6 +101,8 @@ static void __dump_folio(struct folio *folio, struct page *page,
>>  		is_migrate_cma_folio(folio, pfn) ? " CMA" : "");
>>  	if (page_has_type(&folio->page))
>>  		pr_warn("page_type: %pGt\n", &folio->page.page_type);
>> +	else if (folio_test_slab(folio))
>> +		pr_warn("cache flags: %pGs\n", &((struct slab *)&folio->page)->slab_cache->flags);
>> 
> 
> ... because page_has_type() is now true for slab; there is no more
> PG_slab.  I think you also want:
> 
> 	folio_slab(folio)->slab_cache->flags

I didn't notice your other patch about removing PG_slab; it pretty much solves
this issue and much more.
(I had created these patches a few weeks ago.)

> Anyway, we have print_slab_info() which is currently static in slub.c.
> Maybe that needs to become non-static and dump_page() should call that
> for slabs?

Thank you for the suggestions.

print_slab_info() has a slightly different output string format, which does not
match with the dump_page() output style.
Adding it as-it-is looks a bit weird to me.
Other than that, I think it may be useful to print it (which would happen only
when SLAB_DEBUG is enabled).

Also: print_slab_info() is printing the folio's flags. Maybe that needs a change?

--
Sukrit