Re: [PATCH v3 18/21] slab: update overview comments

Suren Baghdasaryan
2026-01-21T20:58:41Z

On Fri, Jan 16, 2026 at 2:41 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> The changes related to sheaves made the description of locking and other
> details outdated. Update it to reflect current state.
>
> Also add a new copyright line due to major changes.
>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
> mm/slub.c | 141 +++++++++++++++++++++++++++++---------------------------------
> 1 file changed, 67 insertions(+), 74 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 2c522d2bf547..476a279f1a94 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1,13 +1,15 @@
> // SPDX-License-Identifier: GPL-2.0
> /*
> - * SLUB: A slab allocator that limits cache line use instead of queuing
> - * objects in per cpu and per node lists.
> + * SLUB: A slab allocator with low overhead percpu array caches and mostly
> + * lockless freeing of objects to slabs in the slowpath.
> *
> - * The allocator synchronizes using per slab locks or atomic operations
> - * and only uses a centralized lock to manage a pool of partial slabs.
> + * The allocator synchronizes using spin_trylock for percpu arrays in the
> + * fastpath, and cmpxchg_double (or bit spinlock) for slowpath freeing.
> + * Uses a centralized lock to manage a pool of partial slabs.
> *
> * (C) 2007 SGI, Christoph Lameter
> * (C) 2011 Linux Foundation, Christoph Lameter
> + * (C) 2025 SUSE, Vlastimil Babka
> */
>
> #include <linux/mm.h>
> @@ -53,11 +55,13 @@
>
> /*
> * Lock order:
> - * 1. slab_mutex (Global Mutex)
> - * 2. node->list_lock (Spinlock)
> - * 3. kmem_cache->cpu_slab->lock (Local lock)
> - * 4. slab_lock(slab) (Only on some arches)
> - * 5. object_map_lock (Only for debugging)
> + * 0. cpu_hotplug_lock
> + * 1. slab_mutex (Global Mutex)
> + * 2a. kmem_cache->cpu_sheaves->lock (Local trylock)
> + * 2b. node->barn->lock (Spinlock)
> + * 2c. node->list_lock (Spinlock)
> + * 3. slab_lock(slab) (Only on some arches)
> + * 4. object_map_lock (Only for debugging)
> *
> * slab_mutex
> *
> @@ -78,31 +82,38 @@
> * C. slab->objects -> Number of objects in slab
> * D. slab->frozen -> frozen state
> *
> - * Frozen slabs
> + * SL_partial slabs
> + *
> + * Slabs on node partial list have at least one free object. A limited number
> + * of slabs on the list can be fully free (slab->inuse == 0), until we start
> + * discarding them. These slabs are marked with SL_partial, and the flag is
> + * cleared while removing them, usually to grab their freelist afterwards.
> + * This clearing also exempts them from list management. Please see
> + * __slab_free() for more details.
> *
> - * If a slab is frozen then it is exempt from list management. It is
> - * the cpu slab which is actively allocated from by the processor that
> - * froze it and it is not on any list. The processor that froze the
> - * slab is the one who can perform list operations on the slab. Other
> - * processors may put objects onto the freelist but the processor that
> - * froze the slab is the only one that can retrieve the objects from the
> - * slab's freelist.
> + * Full slabs
> *
> - * CPU partial slabs
> + * For caches without debugging enabled, full slabs (slab->inuse ==
> + * slab->objects and slab->freelist == NULL) are not placed on any list.
> + * The __slab_free() freeing the first object from such a slab will place
> + * it on the partial list. Caches with debugging enabled place such slab
> + * on the full list and use different allocation and freeing paths.
> + *
> + * Frozen slabs
> *
> - * The partially empty slabs cached on the CPU partial list are used
> - * for performance reasons, which speeds up the allocation process.
> - * These slabs are not frozen, but are also exempt from list management,
> - * by clearing the SL_partial flag when moving out of the node
> - * partial list. Please see __slab_free() for more details.
> + * If a slab is frozen then it is exempt from list management. It is used to
> + * indicate a slab that has failed consistency checks and thus cannot be
> + * allocated from anymore - it is also marked as full. Any previously
> + * allocated objects will be simply leaked upon freeing instead of attempting
> + * to modify the potentially corrupted freelist and metadata.
> *
> * To sum up, the current scheme is:
> - * - node partial slab: SL_partial && !frozen
> - * - cpu partial slab: !SL_partial && !frozen
> - * - cpu slab: !SL_partial && frozen
> - * - full slab: !SL_partial && !frozen
> + * - node partial slab: SL_partial && !full && !frozen
> + * - taken off partial list: !SL_partial && !full && !frozen
> + * - full slab, not on any list: !SL_partial && full && !frozen
> + * - frozen due to inconsistency: !SL_partial && full && frozen
> *
> - * list_lock
> + * node->list_lock (spinlock)
> *
> * The list_lock protects the partial and full list on each node and
> * the partial slab counter. If taken then no new slabs may be added or
> @@ -112,47 +123,46 @@
> *
> * The list_lock is a centralized lock and thus we avoid taking it as
> * much as possible. As long as SLUB does not have to handle partial
> - * slabs, operations can continue without any centralized lock. F.e.
> - * allocating a long series of objects that fill up slabs does not require
> - * the list lock.
> + * slabs, operations can continue without any centralized lock.
> *
> * For debug caches, all allocations are forced to go through a list_lock
> * protected region to serialize against concurrent validation.
> *
> - * cpu_slab->lock local lock
> + * cpu_sheaves->lock (local_trylock)
> *
> - * This locks protect slowpath manipulation of all kmem_cache_cpu fields
> - * except the stat counters. This is a percpu structure manipulated only by
> - * the local cpu, so the lock protects against being preempted or interrupted
> - * by an irq. Fast path operations rely on lockless operations instead.
> + * This lock protects fastpath operations on the percpu sheaves. On !RT it
> + * only disables preemption and does no atomic operations. As long as the main
> + * or spare sheaf can handle the allocation or free, there is no other
> + * overhead.
> *
> - * On PREEMPT_RT, the local lock neither disables interrupts nor preemption
> - * which means the lockless fastpath cannot be used as it might interfere with
> - * an in-progress slow path operations. In this case the local lock is always
> - * taken but it still utilizes the freelist for the common operations.
> + * node->barn->lock (spinlock)
> *
> - * lockless fastpaths
> + * This lock protects the operations on per-NUMA-node barn. It can quickly
> + * serve an empty or full sheaf if available, and avoid more expensive refill
> + * or flush operation.
> *
> - * The fast path allocation (slab_alloc_node()) and freeing (do_slab_free())
> - * are fully lockless when satisfied from the percpu slab (and when
> - * cmpxchg_double is possible to use, otherwise slab_lock is taken).
> - * They also don't disable preemption or migration or irqs. They rely on
> - * the transaction id (tid) field to detect being preempted or moved to
> - * another cpu.
> + * Lockless freeing
> + *
> + * Objects may have to be freed to their slabs when they are from a remote
> + * node (where we want to avoid filling local sheaves with remote objects)
> + * or when there are too many full sheaves. On architectures supporting
> + * cmpxchg_double this is done by a lockless update of slab's freelist and
> + * counters, otherwise slab_lock is taken. This only needs to take the
> + * list_lock if it's a first free to a full slab, or when there are too many
> + * fully free slabs and some need to be discarded.
> *
> * irq, preemption, migration considerations
> *
> - * Interrupts are disabled as part of list_lock or local_lock operations, or
> + * Interrupts are disabled as part of list_lock or barn lock operations, or
> * around the slab_lock operation, in order to make the slab allocator safe
> * to use in the context of an irq.
> + * Preemption is disabled as part of local_trylock operations.
> + * kmalloc_nolock() and kfree_nolock() are safe in NMI context but see
> + * their limitations.
> *
> - * In addition, preemption (or migration on PREEMPT_RT) is disabled in the
> - * allocation slowpath, bulk allocation, and put_cpu_partial(), so that the
> - * local cpu doesn't change in the process and e.g. the kmem_cache_cpu pointer
> - * doesn't have to be revalidated in each section protected by the local lock.
> - *
> - * SLUB assigns one slab for allocation to each processor.
> - * Allocations only occur from these slabs called cpu slabs.
> + * SLUB assigns two object arrays called sheaves for caching allocation and

s/allocation/allocations

> + * frees on each cpu, with a NUMA node shared barn for balancing between cpus.
> + * Allocations and frees are primarily served from these sheaves.
> *
> * Slabs with free elements are kept on a partial list and during regular
> * operations no list for full slabs is used. If an object in a full slab is
> @@ -160,25 +170,8 @@
> * We track full slabs for debugging purposes though because otherwise we
> * cannot scan all objects.
> *
> - * Slabs are freed when they become empty. Teardown and setup is
> - * minimal so we rely on the page allocators per cpu caches for
> - * fast frees and allocs.
> - *
> - * slab->frozen The slab is frozen and exempt from list processing.
> - * This means that the slab is dedicated to a purpose
> - * such as satisfying allocations for a specific
> - * processor. Objects may be freed in the slab while
> - * it is frozen but slab_free will then skip the usual
> - * list operations. It is up to the processor holding
> - * the slab to integrate the slab into the slab lists
> - * when the slab is no longer needed.
> - *
> - * One use of this flag is to mark slabs that are
> - * used for allocations. Then such a slab becomes a cpu
> - * slab. The cpu slab may be equipped with an additional
> - * freelist that allows lockless access to
> - * free objects in addition to the regular freelist
> - * that requires the slab lock.
> + * Slabs are freed when they become empty. Teardown and setup is minimal so we
> + * rely on the page allocators per cpu caches for fast frees and allocs.
> *
> * SLAB_DEBUG_FLAGS Slab requires special handling due to debug
> * options set. This moves slab handling out of
>
> --
> 2.52.0
>