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

[Suren Baghdasaryan <surenb@google.com>]

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 details outdated.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
>