[PATCH 4/4] maple_tree: fix potential allocation failure even has memory

[Wei Yang]

Related fix has been posted, but not merged yet.

lkml.kernel.org/r/20240626160631.3636515-1-Liam.Howlett at oracle.com

May drop this one.

On Tue, Sep 24, 2024 at 12:39:54PM +0000, Wei Yang wrote:
>We got an rare case when mas_node_count() would fail even there is
>enough memory.
>
>The reason is the maple_alloc grows downward. And when hit a full
>maple_alloc, the max_req would be 0. This leads to mt_alloc_bulk()
>return 0, which means failure here.
>
>For example, here is the test code:
>
>	expect = MAPLE_ALLOC_SLOTS + 1;
>	mas_node_count(&ms, expect);
>	pr_info("expect %d allocated %lu\n", expect, mas_allocated(&ms));
>	expect = MAPLE_ALLOC_SLOTS * 2 + 2;
>	mas_node_count(&ms, expect);
>	pr_info("expect %d allocated %lu\n", expect, mas_allocated(&ms));
>
>We will get the following output, which shows we fail to allocate the
>required number of nodes.
>
>	expect 31 allocated 31
>        expect 62 allocated 61
>
>The straight forward way to fix it is go down one level more.
>
>Signed-off-by: Wei Yang <richard.weiyang at gmail.com>
>CC: Liam R. Howlett <Liam.Howlett at Oracle.com>
>CC: Sidhartha Kumar <sidhartha.kumar at oracle.com>
>---
> lib/maple_tree.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
>diff --git a/lib/maple_tree.c b/lib/maple_tree.c
>index 1cbc5f7ca40d..dd33d0793dd1 100644
>--- a/lib/maple_tree.c
>+++ b/lib/maple_tree.c
>@@ -1253,8 +1253,10 @@ static inline void mas_alloc_nodes(struct ma_state *mas, gfp_t gfp)
> 	}
> 
> 	node = mas->alloc;
>-	while (requested) {
>+	for (; requested; node = node->slot[0]) {
> 		max_req = MAPLE_ALLOC_SLOTS - node->node_count;
>+		if (unlikely(!max_req))
>+			continue;
> 		slots = (void **)&node->slot[node->node_count];
> 		max_req = min(requested, max_req);
> 		count = mt_alloc_bulk(gfp, max_req, slots);
>@@ -1268,7 +1270,6 @@ static inline void mas_alloc_nodes(struct ma_state *mas, gfp_t gfp)
> 
> 		node->node_count += count;
> 		allocated += count;
>-		node = node->slot[0];
> 		requested -= count;
> 	}
> 	mas->alloc->total = allocated;
>-- 
>2.34.1

-- 
Wei Yang
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