linux-kernel - Re: [PATCH v3 3/7] mm/vmalloc.c: allow vread() to read out vm_map

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Date:   Mon, 16 Jan 2023 12:50:50 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org, urezki@...il.com,
        lstoakes@...il.com, stephen.s.brennan@...cle.com,
        willy@...radead.org, akpm@...ux-foundation.org, hch@...radead.org
Subject: Re: [PATCH v3 3/7] mm/vmalloc.c: allow vread() to read out
 vm_map_ram areas

On Fri, Jan 13, 2023 at 11:19:17AM +0800, Baoquan He wrote:
> Currently, vread can read out vmalloc areas which is associated with
> a vm_struct. While this doesn't work for areas created by vm_map_ram()
> interface because it doesn't have an associated vm_struct. Then in vread(),
> these areas are all skipped.
> 
> Here, add a new function vmap_ram_vread() to read out vm_map_ram areas.
> The area created with vmap_ram_vread() interface directly can be handled
> like the other normal vmap areas with aligned_vread(). While areas
> which will be further subdivided and managed with vmap_block need
> carefully read out page-aligned small regions and zero fill holes.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  mm/vmalloc.c | 80 +++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 73 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index ab4825050b5c..13875bc41e27 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3544,6 +3544,65 @@ static int aligned_vread(char *buf, char *addr, unsigned long count)
>  	return copied;
>  }
>  
> +static void vmap_ram_vread(char *buf, char *addr, int count, unsigned long flags)
> +{
> +	char *start;
> +	struct vmap_block *vb;
> +	unsigned long offset;
> +	unsigned int rs, re, n;
> +
> +	/*
> +	 * If it's area created by vm_map_ram() interface directly, but
> +	 * not further subdividing and delegating management to vmap_block,
> +	 * handle it here.
> +	 */
> +	if (!(flags & VMAP_BLOCK)) {
> +		aligned_vread(buf, addr, count);
> +		return;
> +	}
> +
> +	/*
> +	 * Area is split into regions and tracked with vmap_block, read out
> +	 * each region and zero fill the hole between regions.
> +	 */
> +	vb = xa_load(&vmap_blocks, addr_to_vb_idx((unsigned long)addr));
> +
> +	spin_lock(&vb->lock);
> +	if (bitmap_empty(vb->used_map, VMAP_BBMAP_BITS)) {
>
CPU-X invokes free_vmap_block() whereas we take the vb->lock and do
some manipulations with vb that might be already freed over RCU-core.

Should we protect it by the rcu_read_lock() also here?

--
Uladzislau Rezki