Re: [PATCH] mm: slub: fix conversion of freelist_corrupted()

[Eugeniu Rosca <erosca@de.adit-jv.com>]

Dear Andrew,

On Tue, Aug 11, 2020 at 01:49:09PM -0700, Andrew Morton wrote:
> On Tue, 11 Aug 2020 14:46:56 +0200 Eugeniu Rosca <erosca@de.adit-jv.com> wrote:
> 
> > Commit 52f23478081ae0 ("mm/slub.c: fix corrupted freechain in
> > deactivate_slab()") suffered an update when picked up from LKML [1].
> > 
> > Specifically, relocating 'freelist = NULL' into 'freelist_corrupted()'
> > created a no-op statement. Fix it by sticking to the behavior intended
> > in the original patch [1]. Prefer the lowest-line-count solution.
> > 
> > [1] https://lore.kernel.org/linux-mm/20200331031450.12182-1-dongli.zhang@oracle.com/
> > 
> > ...
> >
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -677,7 +677,6 @@ static bool freelist_corrupted(struct kmem_cache *s, struct page *page,
> >  	if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
> >  	    !check_valid_pointer(s, page, nextfree)) {
> >  		object_err(s, page, freelist, "Freechain corrupt");
> > -		freelist = NULL;
> >  		slab_fix(s, "Isolate corrupted freechain");
> >  		return true;
> >  	}
> > @@ -2184,8 +2183,10 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
> >  		 * 'freelist' is already corrupted.  So isolate all objects
> >  		 * starting at 'freelist'.
> >  		 */
> > -		if (freelist_corrupted(s, page, freelist, nextfree))
> > +		if (freelist_corrupted(s, page, freelist, nextfree)) {
> > +			freelist = NULL;
> >  			break;
> > +		}
> >  
> >  		do {
> >  			prior = page->freelist;
> 
> Looks right.
> 
> What are the runtime effects of this change?  In other words, what are
> the end user visible effects of the present defect?

Thank you for the prompt feedback.

The issue popped up as a result of static analysis and code review.
Therefore, I lack any specific runtime behavior example being fixed.
Nevertheless, I think this does not diminish the concern expressed in
the description of the patch.

-- 
Best regards,
Eugeniu Rosca