Re: [PATCH] slab: fix memory leak when refill_sheaf() fails

[Vlastimil Babka]

On 3/11/26 12:16, Harry Yoo wrote:
> On Wed, Mar 11, 2026 at 05:36:17PM +0800, Qing Wang wrote:
>> When refill_sheaf() partially fills one sheaf (e.g., fills 5 objects
>> but need to fill 10), it will update sheaf->size and return -ENOMEM.
>> However, the callers (alloc_full_sheaf() and __pcs_replace_empty_main())
>> directly call free_empty_sheaf() on failure, which only does kfree(sheaf),
>> causing the partially allocated objects memory in sheaf->objects[] leaked.
> 
> Nice catch, thanks!

Indeed, thanks!

> Probably the need to fail new_slab() made it quite hard to trigger and notice.

Agreed.

>> Fix this by calling sheaf_flush_unused() before free_empty_sheaf() to
>> free objects of sheaf->objects[]. And also add a WARN_ON() in
>> free_empty_sheaf() to catch any future cases where a non-empty sheaf is
>> being freed.
>> 
>> Fixes: 2d517aa09bbc ("slab: add opt-in caching layer of percpu sheaves")

Actually I think that commit was fine as it was using bulk alloc to refill
and that was undoing any partial successes. I think this one is correct and
replaced it so:

Fixes: ed30c4adfc2b ("slab: add optimized sheaf refill from partial list")

> I think we need to add Cc: stable@xxxxxxxxxxxxxxx

And therefore we don't, unless I'm mistaken.

>> Signed-off-by: Qing Wang <wangqing7171@xxxxxxxxx>
>> ---
>>  mm/slub.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>> 
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 20cb4f3b636d..73b2cfd0e123 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2797,6 +2797,7 @@ static void free_empty_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf)
>>  	if (s->flags & SLAB_KMALLOC)
>>  		mark_obj_codetag_empty(sheaf);
>>  
>> +	WARN_ON(sheaf->size > 0);
> 
> nit: perhaps VM_WARN_ON_ONCE(); will be enough?

Yep replaced it too.

Added to slab/for-next-fixes, thanks!

> Otherwise looks good to me, so:
> Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
>