Re: [RFC v1 5/5] maple_tree: replace preallocation with slub percpu array prefill

Liam R. Howlett
2023-08-08T14:38:33Z

* Vlastimil Babka <vbabka@suse.cz> [230808 05:53]:
> With the percpu array we can try not doing the preallocations in maple
> tree, and instead make sure the percpu array is prefilled, and using
> GFP_ATOMIC in places that relied on the preallocation (in case we miss
> or fail trylock on the array), i.e. mas_store_prealloc(). For now simply
> add __GFP_NOFAIL there as well.
>
> First I tried to change mas_node_count_gfp() to not preallocate anything
> anywhere, but that lead to warns and panics, even though the other
> caller mas_node_count() uses GFP_NOWAIT | __GFP_NOWARN so it has no
> guarantees... So I changed just mas_preallocate(). I let it still to
> truly preallocate a single node, but maybe it's not necessary?

Ah, yes. I added a check to make sure we didn't allocate more nodes
when using preallocations. This check is what you are hitting when you
don't allocate anything. This is tracked in mas_flags by
setting/clearing MA_STATE_PREALLOC. Good news, that check works!

> ---
> lib/maple_tree.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 7a8e7c467d7c..5a209d88c318 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -5534,7 +5534,12 @@ void mas_store_prealloc(struct ma_state *mas, void *entry)
>
> mas_wr_store_setup(&wr_mas);
> trace_ma_write(__func__, mas, 0, entry);
> +
> +retry:
> mas_wr_store_entry(&wr_mas);
> + if (unlikely(mas_nomem(mas, GFP_ATOMIC | __GFP_NOFAIL)))
> + goto retry;
> +
> MAS_WR_BUG_ON(&wr_mas, mas_is_err(mas));
> mas_destroy(mas);
> }
> @@ -5550,9 +5555,10 @@ EXPORT_SYMBOL_GPL(mas_store_prealloc);
> int mas_preallocate(struct ma_state *mas, gfp_t gfp)
> {
> int ret;
> + int count = 1 + mas_mt_height(mas) * 3;
>
> - mas_node_count_gfp(mas, 1 + mas_mt_height(mas) * 3, gfp);
> - mas->mas_flags |= MA_STATE_PREALLOC;
> + mas_node_count_gfp(mas, 1, gfp);
> + kmem_cache_prefill_percpu_array(maple_node_cache, count, gfp);
> if (likely(!mas_is_err(mas)))
> return 0;
>
> --
> 2.41.0
>