linux-kernel - [PATCH v2] mm: vmalloc: Check if a hash-index is in cpu_possible

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Message-Id: <20240626140330.89836-1-urezki@gmail.com>
Date: Wed, 26 Jun 2024 16:03:30 +0200
From: "Uladzislau Rezki (Sony)" <urezki@...il.com>
To: linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Baoquan He <bhe@...hat.com>,
	Hailong Liu <hailong.liu@...o.com>,
	Christoph Hellwig <hch@...radead.org>,
	Nick Bowler <nbowler@...conx.ca>,
	Uladzislau Rezki <urezki@...il.com>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: [PATCH v2] mm: vmalloc: Check if a hash-index is in cpu_possible_mask

The problem is that there are systems where cpu_possible_mask
has gaps between set CPUs, for example SPARC. In this scenario
addr_to_vb_xa() hash function can return an index which accesses
to not-possible and not setup CPU area using per_cpu() macro.

A per-cpu vmap_block_queue is also used as hash table, incorrectly
assuming the cpu_possible_mask has no gaps. Fix it by adjusting an
index to a next possible CPU.

v1 -> v2:
 - update a commit message.

Fixes: 062eacf57ad9 ("mm: vmalloc: remove a global vmap_blocks xarray")
Reported-by: Nick Bowler <nbowler@...conx.ca>
Closes: https://lore.kernel.org/linux-kernel/ZntjIE6msJbF8zTa@MiWiFi-R3L-srv/T/
Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
---
 mm/vmalloc.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index b4c42da9f3901..6b783baf12a14 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2544,7 +2544,15 @@ static DEFINE_PER_CPU(struct vmap_block_queue, vmap_block_queue);
 static struct xarray *
 addr_to_vb_xa(unsigned long addr)
 {
-	int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
+	int index = (addr / VMAP_BLOCK_SIZE) % nr_cpu_ids;
+
+	/*
+	 * Please note, nr_cpu_ids points on a highest set
+	 * possible bit, i.e. we never invoke cpumask_next()
+	 * if an index points on it which is nr_cpu_ids - 1.
+	 */
+	if (!cpu_possible(index))
+		index = cpumask_next(index, cpu_possible_mask);
 
 	return &per_cpu(vmap_block_queue, index).vmap_blocks;
 }
-- 
2.39.2