linux-kernel - Re: [PATCH v8 22/23] maple_tree: Add single node allocation support to maple state

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Message-ID: <6b983596-e521-4714-8a5b-758da76eb1c7@suse.cz>
Date: Mon, 29 Sep 2025 09:39:57 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>,
 Uladzislau Rezki <urezki@...il.com>,
 Sidhartha Kumar <sidhartha.kumar@...cle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
 maple-tree@...ts.infradead.org
Subject: Re: [PATCH v8 22/23] maple_tree: Add single node allocation support
 to maple state

On 9/27/25 03:17, Suren Baghdasaryan wrote:
> On Wed, Sep 10, 2025 at 1:01 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>>
>> From: "Liam R. Howlett" <Liam.Howlett@...cle.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@...cle.com>
>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>> ---
>>  include/linux/maple_tree.h       |  4 +++-
>>  lib/maple_tree.c                 | 47 +++++++++++++++++++++++++++++++++++-----
>>  tools/testing/radix-tree/maple.c |  9 ++++++--
>>  3 files changed, 51 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/linux/maple_tree.h b/include/linux/maple_tree.h
>> index 166fd67e00d882b1e6de1f80c1b590bba7497cd3..562a1e9e5132b5b1fa8f8402a7cadd8abb65e323 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 */

Replacing with: /* A single allocated node for fast path writes */

since I'm touching it anyway due to previous patch fixup.

>>
>> @@ -1093,9 +1100,34 @@ 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;
> 
> Hmm, I don't get the above logic. One node is requested and instead of
> using possibly available mas->alloc, we jump to using mas->sheaf and
> freeing mas->alloc... That does not sound efficient. What am I
> missing?

I'm not changing it now due to merge window, only cosmetic changes and r-b
tags. Can leave it to a follow-up optimization.