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

[Dongli Zhang <dongli.zhang@oracle.com>]

On 8/14/20 12:46 AM, Eugeniu Rosca wrote:
> Hello Dongli,
> 
> On Thu, Aug 13, 2020 at 11:57:51PM -0700, Dongli Zhang wrote:
>> On 8/11/20 5:46 AM, Eugeniu Rosca 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://urldefense.com/v3/__https://lore.kernel.org/linux-mm/20200331031450.12182-1-dongli.zhang@oracle.com/__;!!GqivPVa7Brio!LkxH4qJ3WzKnO_nmONLWV-HAougEaefnp8UnI6qC_8j0SS9_9fkO6bOe68flixlQzx8$ 
>>>
>>> Fixes: 52f23478081ae0 ("mm/slub.c: fix corrupted freechain in deactivate_slab()")
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Dongli Zhang <dongli.zhang@oracle.com>
>>> Cc: <stable@vger.kernel.org>
>>> Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
>>> ---
>>>  mm/slub.c | 5 +++--
>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/slub.c b/mm/slub.c
>>> index 68c02b2eecd9..9a3e963b02a3 100644
>>> --- 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;
>>
>> This is good to me.
>>
>> However, this would confuse people when CONFIG_SLUB_DEBUG is not defined.
>>
>> While reading the source code, people may be curious why to reset freelist when
>> CONFIG_SLUB_DEBUG is even not defined.
> 
> This is a fair point. To address it, the `freelist = NULL` assignment
> should be then moved into the body of freelist_corrupted(). If no
> concerns on that, I will soon push a v2 implementing this proposal.
> 

I do have have concern with that if that can make both of static analysis tool
and the people reading code happy :)

Thank you very much!

Dongli Zhang