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

[Vlastimil Babka <vbabka@suse.cz>]

On 2/24/25 03:43, Hyesoo Yu wrote:
> On Fri, Feb 21, 2025 at 05:16:01PM +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@samsung.com>
>> > Change-Id: I73cf76c110eed62506643913517c957c05a29520
>> > ---
>> >  mm/slub.c | 29 ++++++++++++++---------------
>> >  1 file changed, 14 insertions(+), 15 deletions(-)
>> > 
>> 
>> > @@ -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);
>> > +	if (slab_obj_print)
>> > +		object_err(s, slab, object, buf);
>> 
>> 
>> Wait, I think it's better to keep printing "%s overwritten" regardless
>> of slab_obj_print and only call __slab_err() if slab_obj_print == true
>> as discussed here [1]? Becuase in case there are multiple errors,
>> users should know.
>> 
>> [1] https://lore.kernel.org/all/2ff52c5e-4b6b-4b3d-9047-f00967315d3e@suse.cz
>> 
> 
> Hi,
> 
> __slab_err() doesn't include print_trainer(). It needs object_err().

print_trailer() could be used directly?

> How about including the specific error name 'what' to pr_err ?
> And then object_err would print "Object corrupt" at the beginning once
> without buf like below.

Could also work.

> 	if (slab_obj_print)
> 		object_err(s, slab, object, "Object corrupt");
> 
> 	pr_err("[%s] 0x%p-0x%p @offset=%tu. First byte 0x%x instead of 0x%x\n",
>  	       what, fault, end - 1, fault - addr, fault[0], value);

Probably in opposite order so object_err doesn't panic_on_warn before the
pr_err?

> Thanks,
> Regards.
>> -- 
>> Cheers,
>> Harry
>> 
> 
>