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Message-ID: <6rvzsp6i6p6kc63acbg7hmqlsfx5htvyg5rax3llrauwwyzg4e@f436k2inorfe>
Date: Tue, 26 Aug 2025 11:10:49 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, 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>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
maple-tree@...ts.infradead.org
Subject: Re: [PATCH v5 13/14] maple_tree: Add single node allocation support
to maple state
* Suren Baghdasaryan <surenb@...gle.com> [250822 16:25]:
> On Wed, Jul 23, 2025 at 6:35 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 ++++++++++++++++++++++++++++++++++++++++------
> > 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?
This would be the case if we have one node allocated and requested more
than one node. That is, a chained request for nodes that ends up having
the alloc set and requesting a sheaf.
> Do we really need to free mas->alloc
> here or it can be reused for the next 1-node allocation?
Most calls end in mas_destroy() so that won't happen today.
We could reduce the number of allocations requested to the sheaf and let
the code find the mas->alloc first and use that.
But remember, we are getting into this situation where code did a
mas_preallocate() then figured they needed to do something else (error
recovery, or changed the vma flags and now it can merge..) and will now
need additional nodes. So this is a rare case, so I figured just free
it was the safest thing.
> > + 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
> >
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