[PATCH] mm: vmalloc: Check if a hash-index is in cpu_possible_mask

[PATCH] mm: vmalloc: Check if a hash-index is in cpu_possible_mask

[Uladzislau Rezki (Sony)]

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 assuming
that incorrectly assumes the cpu_possible_mask has not gaps.

Fix it by adjusting an index to a next possible CPU.

Fixes: 062eacf57ad9 ("mm: vmalloc: remove a global vmap_blocks xarray")
Reported-by: Nick Bowler <nbowler@draconx.ca>
Closes: https://lore.kernel.org/linux-kernel/ZntjIE6msJbF8zTa@MiWiFi-R3L-srv/T/
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.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

Re: [PATCH] mm: vmalloc: Check if a hash-index is in cpu_possible_mask

[Baoquan He]

On 06/26/24 at 01:43pm, Uladzislau Rezki (Sony) wrote:
> 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 assuming
                                                        ~~~~~
> that incorrectly assumes the cpu_possible_mask has not gaps.
                   ~~~~~~
                         Typo of duplicated word?

Other than above nit, this looks good to me. Thanks.

Reviewed-by: Baoquan He <bhe@redhat.com>

> 
> Fix it by adjusting an index to a next possible CPU.
> 
> Fixes: 062eacf57ad9 ("mm: vmalloc: remove a global vmap_blocks xarray")
> Reported-by: Nick Bowler <nbowler@draconx.ca>
> Closes: https://lore.kernel.org/linux-kernel/ZntjIE6msJbF8zTa@MiWiFi-R3L-srv/T/
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.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
>