Re: [PATCH v2] slab: allow sheaves for SLAB_NOLEAKTRACE caches

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Nov 13, 2026 at 06:36:56AM -0800, Breno Leitao wrote:
> This is a patch that makes kmemleak cheaper to run, by letting
> kmemleak's own metadata cache use a _new_ sheaves type. The flag only
> makes that possible without recursion.
> 
> The cost today:
>   * every allocation kmemleak tracks needs one kmemleak_object from
>     object_cache. An RCU callback frees it later.
> 
> Why that's slow:
>   * object_cache has no sheaves, so each of those allocations and frees
>     goes to the shared slab lists under n->list_lock.
>      - On meta's fleet kmemleak kernels, list_lock is the second most
>        contended lock, after kmemleak_lock.
>      - About 88% of that wait is RCU callbacks freeing kmemleak objects.
> 
> Why sheaves aren't allowed:
>   * a sheaf is a kmalloc object. Tracking it makes kmemleak allocate
>     another kmemleak_object, which needs a sheaf, and so on.
> 
> When I reported the recursion in [1], I suggested keeping
> SLAB_NOLEAKTRACE caches away from sheaves, and commit e47c897a2949
> ("slab: add sheaves to most caches") did that. It stopped the
> recursion, but it took the sheaves away from the whole cache, although
> only the kmemleak tracking of the sheaf itself recursed.
> 
> Commit 17c38c88294d ("slab: remove cpu (partial) slabs usage from
> allocation paths"), from the same series, then left these caches with
> no per-CPU caching at all.
> 
> In hindsight, skipping kmemleak only for the
> sheaf itself, as the new SLAB_ALLOC_NO_LEAKTRACE does, would have been
> the better fix.
> 
> The sheaf is allocated with SLAB_ALLOC_NO_RECURSE as well, like kmalloc
> sheaves. Without it, a backing kmalloc cache that needs a new sheaf of
> its own allocates it with SLAB_ALLOC_NO_LEAKTRACE too. kmemleak then
> does not track that sheaf, and can report objects queued in it by
> kfree_rcu() as leaks.
> 
> SLAB_ALLOC_NO_RECURSE also skips allocation tagging, so mark the codetag
> of these sheaves empty on free, as is done for kmalloc sheaves.
> 
> Link: https://lore.kernel.org/all/aW_NK2NXVgtuzCVH@gmail.com/ [1]
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
> Changes in v2:
> - Fix sashiko findings:
>   - Set SLAB_ALLOC_NO_RECURSE along with SLAB_ALLOC_NO_LEAKTRACE, so the
>     flag stays off the kmalloc cache's own sheaf allocation, per Catalin
>     Marinas.
>   - Mark the codetag of SLAB_NOLEAKTRACE sheaves empty on free, per
>     Catalin Marinas.
> - Link to v1: https://patch.msgid.link/20261112-kmemleak_sheaves-v1-1-e40e3d73b253@debian.org

v2 looks fine, so my reviewed-by stands. Thanks for respinning.

-- 
Catalin