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

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Date: Mon, 27 May 2024 10:46:19 +0000
From: "Sukrit.Bhatnagar@...y.com" <Sukrit.Bhatnagar@...y.com>
To: Matthew Wilcox <willy@...radead.org>
CC: Petr Mladek <pmladek@...e.com>, Steven Rostedt <rostedt@...dmis.org>,
        Andy
 Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes
	<linux@...musvillemoes.dk>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Jonathan Corbet <corbet@....net>, Christoph Lameter <cl@...ux.com>,
        Pekka
 Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo
 Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-trace-kernel@...r.kernel.org" <linux-trace-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/2] mm: debug: print correct information for slab folios

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