linux-kernel - Re: [PATCH v3 1/2] mm: slub: Print the broken data before restoring slub.

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Message-ID: <20250220214904.GA1820313@tiffany>
Date: Fri, 21 Feb 2025 06:49:04 +0900
From: Hyesoo Yu <hyesoo.yu@...sung.com>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: janghyuck.kim@...sung.com, vbabka@...e.cz, 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>, Roman Gushchin <roman.gushchin@...ux.dev>,
	Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] mm: slub: Print the broken data before restoring
 slub.

On Thu, Feb 20, 2025 at 08:01:57PM +0900, Harry Yoo wrote:
> On Thu, Feb 20, 2025 at 12:39:43PM +0900, Hyesoo Yu wrote:
> > Previously, the restore occured after printing the object in slub.
> > After commit 47d911b02cbe ("slab: make check_object() more consistent"),
> > the bytes are printed after the restore. This information about the bytes
> > before the restore is highly valuable for debugging purpose.
> > For instance, in a event of cache issue, it displays byte patterns
> > by breaking them down into 64-bytes units. Without this information,
> > we can only speculate on how it was broken. Hence the corrupted regions
> > should be printed prior to the restoration process. However if an object
> > breaks in multiple places, the same log may be output multiple times.
> > Therefore the slub log is reported only once to prevent redundant printing,
> > by sending a parameter indicating whether an error has occurred previously.
> > 
> > Changes in v3:
> > - Change the parameter type of check_bytes_and_report.
> > 
> > Changes in v2:
> > - Instead of using print_section every time on check_bytes_and_report,
> > just print it once for the entire slub object before the restore.
> >
> > Signed-off-by: Hyesoo Yu <hyesoo.yu@...sung.com>
> > Change-Id: I73cf76c110eed62506643913517c957c05a29520
> 
> As previously mentioned by others, Change-Id is not used in Linux
> kernel development.
>

Oops, It is my mistake. I will remove it.

> > ---
> >  mm/slub.c | 29 ++++++++++++++---------------
> >  1 file changed, 14 insertions(+), 15 deletions(-)
> > 
> > diff --git a/mm/slub.c b/mm/slub.c
> > index b3969d63cc04..de62fed12236 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -1192,12 +1192,13 @@ static void restore_bytes(struct kmem_cache *s, char *message, u8 data,
> >  
> >  static pad_check_attributes int
> >  check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
> > -		       u8 *object, char *what,
> > -		       u8 *start, unsigned int value, unsigned int bytes)
> > +		       u8 *object, char *what, u8 *start, unsigned int value,
> > +		       unsigned int bytes, bool slab_obj_print)
> >  {
> >  	u8 *fault;
> >  	u8 *end;
> >  	u8 *addr = slab_address(slab);
> > +	char buf[100];
> >  
> >  	metadata_access_enable();
> >  	fault = memchr_inv(kasan_reset_tag(start), value, bytes);
> > @@ -1212,11 +1213,14 @@ check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
> >  	if (slab_add_kunit_errors())
> >  		goto skip_bug_print;
> >  
> > -	slab_bug(s, "%s overwritten", what);
> >  	pr_err("0x%p-0x%p @offset=%tu. First byte 0x%x instead of 0x%x\n",
> >  					fault, end - 1, fault - addr,
> >  					fault[0], value);
> >  
> > +	scnprintf(buf, 100, "%s overwritten", what);
> 
> How about moving this into the if block and changing 100 to sizeof(buf)?
>

That sounds good. I will change it.

> > +	if (slab_obj_print)
> > +		object_err(s, slab, object, buf);
> > +
> >  skip_bug_print:
> >  	restore_bytes(s, what, value, fault, end);
> >  	return 0;
> > @@ -1279,7 +1283,7 @@ static int check_pad_bytes(struct kmem_cache *s, struct slab *slab, u8 *p)
> >  		return 1;
> >  
> >  	return check_bytes_and_report(s, slab, p, "Object padding",
> > -			p + off, POISON_INUSE, size_from_object(s) - off);
> > +			p + off, POISON_INUSE, size_from_object(s) - off, true);
> >  }
> >  
> >  /* Check the pad bytes at the end of a slab page */
> > @@ -1329,11 +1333,11 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
> >  
> >  	if (s->flags & SLAB_RED_ZONE) {
> >  		if (!check_bytes_and_report(s, slab, object, "Left Redzone",
> > -			object - s->red_left_pad, val, s->red_left_pad))
> > +			object - s->red_left_pad, val, s->red_left_pad, !!ret))
> 
> I think you don't have to add !! to ret.
> Converting from int to _Bool is legal in C99 and it will work as intended. 
> 

Thank you for informing me.
I remove !! to next version.

Thanks,
Regards.

> > 			ret = 0;
> 
> -- 
> Cheers,
> Harry
>