Re: [PATCH v2] mm: slub: call WARN() instead of pr_err on slab_fix.

[Hyesoo Yu <hyesoo.yu@samsung.com>]

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On Wed, Feb 05, 2025 at 06:57:14PM -0800, David Rientjes wrote:
> On Wed, 5 Feb 2025, Vlastimil Babka wrote:
> 
> > On 2/5/25 18:10, David Rientjes wrote:
> > > On Wed, 5 Feb 2025, Hyesoo Yu wrote:
> > > 
> > >> If a slab object is corrupted or an error occurs in its internal
> > >> value, continuing after restoration may cause other side effects.
> > >> At this point, it is difficult to debug because the problem occurred
> > >> in the past. It is better to use WARN() instead of pr_err to catch
> > >> errors at the point of issue because WARN() could trigger panic for
> > >> system debugging when panic_on_warn is enabled. WARN() should be
> > >> called prior to fixing the value because when a panic is triggered by WARN(),
> > >> it allows us to check corrupted data.
> > >> 
> > > 
> > > I think this makes sense, but it doesn't document why the other changes 
> > > are being made, like moving the setting of *freelist to NULL.  This is 
> > > presumably something that you want in the crash dump when 
> > > kernel.panic_on_warn is enabled.  Probably best to call that out, but to 
> > > also indicate what you're relying on in the crash dump to make forward 
> > > progress on in diagnosing the issue.
> > 
> > Well the last sentence of the changelog above says exactly that, no?
> > 
> 
> Sorry, I should have been more clear.  It's unclear in the code why 
> choosing WARN() here is helpful given the stack would be known.  It makes 
> sense to enable kernel.panic_on_warn this way for debugging purposes, but 
> thought it should also carry a comment in the code on the rationale (and 
> the state we're trying to capture in a crash dump) so a future change 
> doesn't go and unravel this for us again.
>

What do you think of this comment ? I will add it in patch v3.

/*
 * slab_fix indicates that the value would be restored even if an error occurs.
 * Or, it is possible to trigger a panic without restoring using WARN() if panic_on_warn
 * is enabled. This can obtain a crash dump at the point of issue to debug.
 * It is advisable not to restore the data before calling slab_fix() to check for corrupted
 * data in the crash dump.
 */

Thanks,
Regards.

> > >> Changes in v2:
> > >> - Replace direct calling with BUG_ON with the use of WARN in slab_fix.
> > >> 
> > >> Signed-off-by: Hyesoo Yu <hyesoo.yu@samsung.com>
> > >> ---
> > >>  mm/slub.c | 10 +++++-----
> > >>  1 file changed, 5 insertions(+), 5 deletions(-)
> > >> 
> > >> diff --git a/mm/slub.c b/mm/slub.c
> > >> index 1f50129dcfb3..ea956cb4b8be 100644
> > >> --- a/mm/slub.c
> > >> +++ b/mm/slub.c
> > >> @@ -1043,7 +1043,7 @@ static void slab_fix(struct kmem_cache *s, char *fmt, ...)
> > >>  	va_start(args, fmt);
> > >>  	vaf.fmt = fmt;
> > >>  	vaf.va = &args;
> > >> -	pr_err("FIX %s: %pV\n", s->name, &vaf);
> > >> +	WARN(1, "FIX %s: %pV\n", s->name, &vaf);
> > >>  	va_end(args);
> > >>  }
> > >>  
> > >> @@ -1106,8 +1106,8 @@ static bool freelist_corrupted(struct kmem_cache *s, struct slab *slab,
> > >>  	if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
> > >>  	    !check_valid_pointer(s, slab, nextfree) && freelist) {
> > >>  		object_err(s, slab, *freelist, "Freechain corrupt");
> > >> -		*freelist = NULL;
> > >>  		slab_fix(s, "Isolate corrupted freechain");
> > >> +		*freelist = NULL;
> > >>  		return true;
> > >>  	}
> > >>  
> > >> @@ -1445,9 +1445,9 @@ static int on_freelist(struct kmem_cache *s, struct slab *slab, void *search)
> > >>  				set_freepointer(s, object, NULL);
> > >>  			} else {
> > >>  				slab_err(s, slab, "Freepointer corrupt");
> > >> +				slab_fix(s, "Freelist cleared");
> > >>  				slab->freelist = NULL;
> > >>  				slab->inuse = slab->objects;
> > >> -				slab_fix(s, "Freelist cleared");
> > >>  				return 0;
> > >>  			}
> > >>  			break;
> > >> @@ -1464,14 +1464,14 @@ static int on_freelist(struct kmem_cache *s, struct slab *slab, void *search)
> > >>  	if (slab->objects != max_objects) {
> > >>  		slab_err(s, slab, "Wrong number of objects. Found %d but should be %d",
> > >>  			 slab->objects, max_objects);
> > >> -		slab->objects = max_objects;
> > >>  		slab_fix(s, "Number of objects adjusted");
> > >> +		slab->objects = max_objects;
> > >>  	}
> > >>  	if (slab->inuse != slab->objects - nr) {
> > >>  		slab_err(s, slab, "Wrong object count. Counter is %d but counted were %d",
> > >>  			 slab->inuse, slab->objects - nr);
> > >> -		slab->inuse = slab->objects - nr;
> > >>  		slab_fix(s, "Object count adjusted");
> > >> +		slab->inuse = slab->objects - nr;
> > >>  	}
> > >>  	return search == NULL;
> > >>  }
> > >> -- 
> > >> 2.48.0
> > >> 
> > >> 
> > 
> > 
> 

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