Re: [PATCH v2] mm/slub: fix lost local objects when bulk remote free batch fills

[hu.shengming]

Vlastimil wrote:
> On 7/6/26 15:39, hu.shengming@xxxxxxxxxx wrote:
> > From: Shengming Hu <hu.shengming@xxxxxxxxxx>
> > 
> > In free_to_pcs_bulk(), when remote_objects[] fills to PCS_BATCH_MAX,
> > the code jumps to flush_remote to free the batch. If all remote entries
> > have already been compacted out of p[] via tail swaps while local objects
> > remain, the flush_remote path returns early since `i < size` no longer
> > holds. The leftover local objects are then neither cached in the sheaf 
> > nor returned to the slab freelist, causing a memory leak.
> > 
> > For illustration:
> >  size = 64, local objects at p[0..31], remote objects at p[32..63]
> >  After scanning all remotes: i = 32, size = 32
> >  p[0..31] local objects are dropped.
> > 
> > Harry pointed out that, although the logic contains a real leak, it does
> > not appear to be triggerable with the current in-tree users. To hit this
> > path, at least PCS_BATCH_MAX objects, currently hardcoded to 32, need to
> > be collected in remote_objects[]. Looking at current kmem_cache_free_bulk()
> > users:
> > 
> > * maple_node has sheaf_capacity = 32
> > * skbuff_head_cache has sheaf_capacity = 28
> > * panthor and msm drivers have sheaf_capacity = 4
> > 
> > The sheaf capacity is, at least for now, derived purely from the object
> > size, with the user-requested capacity used as a minimum. Therefore, among
> > the current users, only maple_node has a sheaf_capacity large enough to
> > reach PCS_BATCH_MAX.
> > 
> > However, for the bug to trigger in maple_node, all objects in the sheaf
> > would have to be from remote nodes. In that case, there would be no local
> > objects left to leak. So this issue was found by code review rather than
> > from a runtime report, and it does not seem to be triggerable by current
> > users.
> > 
> > Still, the bug could become reachable with future users, a different sheaf
> > capacity, or a change to PCS_BATCH_MAX. Fix the logic by freeing a full
> > remote batch in place during the scan and then continuing to process the
> > compacted array. This keeps all local objects on the normal fast path,
> > while the tail path only handles any leftover partial remote batch. The
> > redundant next_remote_batch jump label is removed as well.
> > 
> > Fixes: <989b09b73978>("slab: skip percpu sheaves for remote object freeing")
> 
> Removed the < >
> 

Oh, thanks :)

> > Signed-off-by: Shengming Hu <hu.shengming@xxxxxxxxxx>
> 
> Thanks! I added cc: stable anyway so we avoid unexpected surprises in case
> something else is backported there that exposes the bug. And the fix is
> small enough.
> 
> Merged to slab/for-next-fixes

Sounds good to me. Thanks for taking it!

--
With Best Regards,
Shengming