linux-kernel - Re: [RFC v1 4/5] maple_tree: avoid bulk alloc/free to use percpu array more

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Date:   Tue, 8 Aug 2023 10:29:45 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Peng Zhang <zhangpeng.00@...edance.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC v1 4/5] maple_tree: avoid bulk alloc/free to use percpu
 array more

* Peng Zhang <zhangpeng.00@...edance.com> [230808 07:17]:
> 
> 
> 在 2023/8/8 17:53, Vlastimil Babka 写道:
> > Using bulk alloc/free on a cache with percpu array should not be
> > necessary and the bulk alloc actually bypasses the array (the prefill
> > functionality currently relies on this).
> > 
> > The simplest change is just to convert the respective maple tree
> > wrappers to do a loop of normal alloc/free.
> > ---
> >   lib/maple_tree.c | 11 +++++++++--
> >   1 file changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> > index 1196d0a17f03..7a8e7c467d7c 100644
> > --- a/lib/maple_tree.c
> > +++ b/lib/maple_tree.c
> > @@ -161,12 +161,19 @@ static inline struct maple_node *mt_alloc_one(gfp_t gfp)
> >   static inline int mt_alloc_bulk(gfp_t gfp, size_t size, void **nodes)
> >   {
> > -	return kmem_cache_alloc_bulk(maple_node_cache, gfp, size, nodes);
> > +	int allocated = 0;
> > +	for (size_t i = 0; i < size; i++) {
> > +		nodes[i] = kmem_cache_alloc(maple_node_cache, gfp);
> > +		if (nodes[i])
> If the i-th allocation fails, node[i] will be NULL. This is wrong. We'd
> better guarantee that mt_alloc_bulk() allocates completely successfully,
> or returns 0. The following cases are not allowed:
> nodes: [addr1][addr2][NULL][addr3].

Thanks for pointing this out Peng.

We can handle a lower number than requested being returned, but we
cannot handle the sparse data.

The kmem_cache_alloc_bulk() can return a failure today - leaving the
array to be cleaned by the caller, so if this is changed to a full
success or full fail, then we will also have to change the caller to
handle whatever state is returned if it differs from
kmem_cache_alloc_bulk().

It might be best to return the size already allocated when a failure is
encountered. This will make the caller, mas_alloc_nodes(), request more
nodes.  Only in the case of zero allocations would this be seen as an
OOM event.

Vlastimil, Is the first kmem_cache_alloc() call failing a possibility?
If so, what should be the corrective action?

> > +			allocated++;
> > +	}
> > +	return allocated;
> >   }
> >   static inline void mt_free_bulk(size_t size, void __rcu **nodes)
> >   {
> > -	kmem_cache_free_bulk(maple_node_cache, size, (void **)nodes);
> > +	for (size_t i = 0; i < size; i++)
> > +		kmem_cache_free(maple_node_cache, nodes[i]);
> >   }
> >   static void mt_free_rcu(struct rcu_head *head)