Re: [PATCH v5 3/3] kernel/resource: Fix locking in request_free_mem_region

[David Hildenbrand <david@redhat.com>]

On 19.04.21 09:01, Alistair Popple wrote:
> request_free_mem_region() is used to find an empty range of physical
> addresses for hotplugging ZONE_DEVICE memory. It does this by iterating
> over the range of possible addresses using region_intersects() to see if
> the range is free before calling request_mem_region() to allocate the
> region.
> 
> However the resource_lock is dropped between these two calls meaning by the
> time request_mem_region() is called in request_free_mem_region() another
> thread may have already reserved the requested region. This results in
> unexpected failures and a message in the kernel log from hitting this
> condition:
> 
>          /*
>           * mm/hmm.c reserves physical addresses which then
>           * become unavailable to other users.  Conflicts are
>           * not expected.  Warn to aid debugging if encountered.
>           */
>          if (conflict->desc == IORES_DESC_DEVICE_PRIVATE_MEMORY) {
>                  pr_warn("Unaddressable device %s %pR conflicts with %pR",
>                          conflict->name, conflict, res);
> 
> These unexpected failures can be corrected by holding resource_lock across
> the two calls. This also requires memory allocation to be performed prior
> to taking the lock.
> 
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> ---
>   kernel/resource.c | 45 ++++++++++++++++++++++++++++++++++++++-------
>   1 file changed, 38 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/resource.c b/kernel/resource.c
> index 75f8da722497..e8468e867495 100644
> --- a/kernel/resource.c
> +++ b/kernel/resource.c
> @@ -1801,25 +1801,56 @@ static struct resource *__request_free_mem_region(struct device *dev,
>   {
>   	resource_size_t end, addr;
>   	struct resource *res;
> +	struct region_devres *dr = NULL;
>   
>   	size = ALIGN(size, 1UL << PA_SECTION_SHIFT);
>   	end = min_t(unsigned long, base->end, (1UL << MAX_PHYSMEM_BITS) - 1);
>   	addr = end - size + 1UL;
>   
> +	res = alloc_resource(GFP_KERNEL);
> +	if (!res)
> +		return ERR_PTR(-ENOMEM);
> +
> +	if (dev) {
> +		dr = devres_alloc(devm_region_release,
> +				sizeof(struct region_devres), GFP_KERNEL);
> +		if (!dr) {
> +			free_resource(res);
> +			return ERR_PTR(-ENOMEM);
> +		}
> +	}
> +
> +	write_lock(&resource_lock);
>   	for (; addr > size && addr >= base->start; addr -= size) {
> -		if (region_intersects(addr, size, 0, IORES_DESC_NONE) !=
> +		if (__region_intersects(addr, size, 0, IORES_DESC_NONE) !=
>   				REGION_DISJOINT)
>   			continue;
>   
> -		if (dev)
> -			res = devm_request_mem_region(dev, addr, size, name);
> -		else
> -			res = request_mem_region(addr, size, name);
> -		if (!res)
> -			return ERR_PTR(-ENOMEM);
> +		if (!__request_region_locked(res, &iomem_resource, addr, size,
> +						name, 0))
> +			break;
> +
> +		if (dev) {
> +			dr->parent = &iomem_resource;
> +			dr->start = addr;
> +			dr->n = size;
> +			devres_add(dev, dr);
> +		}
> +
>   		res->desc = IORES_DESC_DEVICE_PRIVATE_MEMORY;
> +		write_unlock(&resource_lock);
> +
> +		/*
> +		 * A driver is claiming this region so revoke any mappings.
> +		 */
> +		revoke_iomem(res);
>   		return res;
>   	}
> +	write_unlock(&resource_lock);
> +
> +	free_resource(res);
> +	if (dr)
> +		devres_free(dr);
>   
>   	return ERR_PTR(-ERANGE);
>   }
> 

LGTM

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb