linux-kernel - Re: [PATCH 1/1] mm/vmalloc.c: Add an error message if two areas overlap

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Date:   Fri, 10 Jul 2020 20:44:06 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Hillf Danton <hdanton@...a.com>,
        Michal Hocko <mhocko@...e.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 1/1] mm/vmalloc.c: Add an error message if two areas
 overlap

On Fri, Jul 10, 2020 at 09:40:42PM +0200, Uladzislau Rezki (Sony) wrote:
> Before triggering a BUG() it would be useful to understand
> how two areas overlap between each other. Print information
> about start/end addresses of both VAs and their addresses.
> 
> For example if both are identical it could mean double free.
> 
> 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 5a2b55c8dd9a..1679b01febcd 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -549,8 +549,13 @@ find_va_links(struct vmap_area *va,
>  		else if (va->va_end > tmp_va->va_start &&
>  				va->va_start >= tmp_va->va_end)
>  			link = &(*link)->rb_right;
> -		else
> +		else {
> +			pr_err("Overlaps: 0x%px(0x%lx-0x%lx), 0x%px(0x%lx-0x%lx)\n",
> +				va, va->va_start, va->va_end, tmp_va,
> +				tmp_va->va_start, tmp_va->va_end);

It might be helpful to have a "vmalloc:" prefix to that string to indicate
where to start searching.  And I don't think we're supposed to use %px
without a really good justification these days.

Can we do without the BUG()?  Maybe just break out and decline to insert
the conflicting range?

>  			BUG();
> +		}
>  	} while (*link);
>  
>  	*parent = &tmp_va->rb_node;
> @@ -1993,6 +1998,9 @@ static void vmap_init_free_space(void)
>  			insert_vmap_area_augment(free, NULL,
>  				&free_vmap_area_root,
>  					&free_vmap_area_list);
> +			insert_vmap_area_augment(free, NULL,
> +				&free_vmap_area_root,
> +					&free_vmap_area_list);

This is surely testing code that you forgot to delete?