Re: [PATCH v5 13/14] maple_tree: Add single node allocation support to maple state

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Jul 23, 2025 at 6:35 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>
>
> The fast path through a write will require replacing a single node in
> the tree.  Using a sheaf (32 nodes) is too heavy for the fast path, so
> special case the node store operation by just allocating one node in the
> maple state.
>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  include/linux/maple_tree.h |  4 +++-
>  lib/maple_tree.c           | 47 ++++++++++++++++++++++++++++++++++++++++------
>  2 files changed, 44 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/maple_tree.h b/include/linux/maple_tree.h
> index 3cf1ae9dde7ce43fa20ae400c01fefad048c302e..61eb5e7d09ad0133978e3ac4b2af66710421e769 100644
> --- a/include/linux/maple_tree.h
> +++ b/include/linux/maple_tree.h
> @@ -443,6 +443,7 @@ struct ma_state {
>         unsigned long min;              /* The minimum index of this node - implied pivot min */
>         unsigned long max;              /* The maximum index of this node - implied pivot max */
>         struct slab_sheaf *sheaf;       /* Allocated nodes for this operation */
> +       struct maple_node *alloc;       /* allocated nodes */
>         unsigned long node_request;
>         enum maple_status status;       /* The status of the state (active, start, none, etc) */
>         unsigned char depth;            /* depth of tree descent during write */
> @@ -491,8 +492,9 @@ struct ma_wr_state {
>                 .status = ma_start,                                     \
>                 .min = 0,                                               \
>                 .max = ULONG_MAX,                                       \
> -               .node_request= 0,                                       \
>                 .sheaf = NULL,                                          \
> +               .alloc = NULL,                                          \
> +               .node_request= 0,                                       \
>                 .mas_flags = 0,                                         \
>                 .store_type = wr_invalid,                               \
>         }
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 3c3c14a76d98ded3b619c178d64099b464a2ca23..9aa782b1497f224e7366ebbd65f997523ee0c8ab 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -1101,16 +1101,23 @@ static int mas_ascend(struct ma_state *mas)
>   *
>   * Return: A pointer to a maple node.
>   */
> -static inline struct maple_node *mas_pop_node(struct ma_state *mas)
> +static __always_inline struct maple_node *mas_pop_node(struct ma_state *mas)
>  {
>         struct maple_node *ret;
>
> +       if (mas->alloc) {
> +               ret = mas->alloc;
> +               mas->alloc = NULL;
> +               goto out;
> +       }
> +
>         if (WARN_ON_ONCE(!mas->sheaf))
>                 return NULL;
>
>         ret = kmem_cache_alloc_from_sheaf(maple_node_cache, GFP_NOWAIT, mas->sheaf);
> -       memset(ret, 0, sizeof(*ret));
>
> +out:
> +       memset(ret, 0, sizeof(*ret));
>         return ret;
>  }
>
> @@ -1121,9 +1128,34 @@ static inline struct maple_node *mas_pop_node(struct ma_state *mas)
>   */
>  static inline void mas_alloc_nodes(struct ma_state *mas, gfp_t gfp)
>  {
> -       if (unlikely(mas->sheaf)) {
> -               unsigned long refill = mas->node_request;
> +       if (!mas->node_request)
> +               return;
> +
> +       if (mas->node_request == 1) {
> +               if (mas->sheaf)
> +                       goto use_sheaf;
> +
> +               if (mas->alloc)
> +                       return;
>
> +               mas->alloc = mt_alloc_one(gfp);
> +               if (!mas->alloc)
> +                       goto error;
> +
> +               mas->node_request = 0;
> +               return;
> +       }
> +
> +use_sheaf:
> +       if (unlikely(mas->alloc)) {

When would this condition happen? Do we really need to free mas->alloc
here or it can be reused for the next 1-node allocation?

> +               mt_free_one(mas->alloc);
> +               mas->alloc = NULL;
> +       }
> +
> +       if (mas->sheaf) {
> +               unsigned long refill;
> +
> +               refill = mas->node_request;
>                 if(kmem_cache_sheaf_size(mas->sheaf) >= refill) {
>                         mas->node_request = 0;
>                         return;
> @@ -5386,8 +5418,11 @@ void mas_destroy(struct ma_state *mas)
>         mas->node_request = 0;
>         if (mas->sheaf)
>                 mt_return_sheaf(mas->sheaf);
> -
>         mas->sheaf = NULL;
> +
> +       if (mas->alloc)
> +               mt_free_one(mas->alloc);
> +       mas->alloc = NULL;
>  }
>  EXPORT_SYMBOL_GPL(mas_destroy);
>
> @@ -6074,7 +6109,7 @@ bool mas_nomem(struct ma_state *mas, gfp_t gfp)
>                 mas_alloc_nodes(mas, gfp);
>         }
>
> -       if (!mas->sheaf)
> +       if (!mas->sheaf && !mas->alloc)
>                 return false;
>
>         mas->status = ma_start;
>
> --
> 2.50.1
>