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Subject: Re: [PATCH] mm/slub: Add taint after the errors are printed
To:     Georgi Djakov <quic_c_gdjako@quicinc.com>,
        akpm@linux-foundation.org, iamjoonsoo.kim@lge.com,
        rientjes@google.com, penberg@kernel.org, cl@linux.com
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org, djakov@kernel.org
References: <1623860738-146761-1-git-send-email-quic_c_gdjako@quicinc.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <4b22baf7-582b-f1ae-a525-046a493ec85f@suse.cz>
Date:   Wed, 16 Jun 2021 18:34:41 +0200
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On 6/16/21 6:25 PM, Georgi Djakov wrote:
> When running the kernel with panic_on_taint, the usual slub debug error
> messages are not being printed when object corruption happens. That's
> because we panic in add_taint(), which is called before printing the
> additional information. This is a bit unfortunate as the error messages
> are actually very useful, especially before a panic. Let's fix this by
> moving add_taint() after the errors are printed on the console.
> 
> Signed-off-by: Georgi Djakov <quic_c_gdjako@quicinc.com>

Makes sense.

While at it, I wonder if we should use LOCKDEP_STILL_OK instead of
LOCKDEP_NOW_UNRELIABLE. Isn't it too pessimistic to assume that some slab's
memory corruption hit some lock state?

> ---
>  mm/slub.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index a8b0daa1a307..ce7b8e4551b5 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -719,8 +719,6 @@ static void slab_bug(struct kmem_cache *s, char *fmt, ...)
>  	pr_err("=============================================================================\n");
>  	pr_err("BUG %s (%s): %pV\n", s->name, print_tainted(), &vaf);
>  	pr_err("-----------------------------------------------------------------------------\n\n");
> -
> -	add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
>  	va_end(args);
>  }
>  
> @@ -801,6 +799,7 @@ void object_err(struct kmem_cache *s, struct page *page,
>  
>  	slab_bug(s, "%s", reason);
>  	print_trailer(s, page, object);
> +	add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
>  }
>  
>  static __printf(3, 4) void slab_err(struct kmem_cache *s, struct page *page,
> @@ -818,6 +817,7 @@ static __printf(3, 4) void slab_err(struct kmem_cache *s, struct page *page,
>  	slab_bug(s, "%s", buf);
>  	print_page_info(page);
>  	dump_stack();
> +	add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
>  }
>  
>  static void init_object(struct kmem_cache *s, void *object, u8 val)
> @@ -869,6 +869,7 @@ static int check_bytes_and_report(struct kmem_cache *s, struct page *page,
>  					fault, end - 1, fault - addr,
>  					fault[0], value);
>  	print_trailer(s, page, object);
> +	add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
>  
>  skip_bug_print:
>  	restore_bytes(s, what, value, fault, end);
>