Re: [PATCH v2 1/2] x86/sgx: Add accounting for tracking overcommit

[Jarkko Sakkinen <jarkko@kernel.org>]

On Fri, Jan 07, 2022 at 10:16:16AM -0800, Kristen Carlson Accardi wrote:
> When the system runs out of enclave memory, SGX can reclaim EPC pages
> by swapping to normal RAM. This normal RAM is allocated via a
> per-enclave shared memory area. The shared memory area is not mapped
> into the enclave or the task mapping it, which makes its memory use
> opaque (including to the OOM killer). Having lots of hard to find
> memory around is problematic, especially when there is no limit.
> 
> Introduce a global counter that can be used to limit the number of pages
> that enclaves are able to consume for backing storage.  This parameter
> is a percentage value that is used in conjunction with the number of
> EPC pages in the system to set a cap on the amount of backing RAM that
> can be consumed.
> 
> The overcommit percentage value is 150, which limits the total number of
> shared memory pages that may be consumed by all enclaves as backing pages
> to 1.5X of EPC pages on the system.
> 
> For example, on an SGX system that has 128MB of EPC, this would
> cap the amount of normal RAM that SGX consumes for its shared memory
> areas at 192MB.
> 
> The SGX overcommit percent works differently than the core VM overcommit
> limit. Enclaves request backing pages one page at a time, and the number
> of in use backing pages that are allowed is a global resource that is
> limited for all enclaves.
> 
> Introduce a pair of functions which can be used by callers when requesting
> backing RAM pages. These functions are responsible for accounting the
> page charges. A request may return an error if the request will cause the
> counter to exceed the backing page cap.
> 
> Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
> ---
>  arch/x86/kernel/cpu/sgx/main.c | 46 ++++++++++++++++++++++++++++++++++
>  arch/x86/kernel/cpu/sgx/sgx.h  |  2 ++
>  2 files changed, 48 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index 2857a49f2335..8a7bfe0d9023 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -43,6 +43,45 @@ static struct sgx_numa_node *sgx_numa_nodes;
>  
>  static LIST_HEAD(sgx_dirty_page_list);
>  
> +/*
> + * Limits the amount of normal RAM that SGX can consume for EPC
> + * overcommit to the total EPC pages * sgx_overcommit_percent / 100
> + */
> +static int sgx_overcommit_percent = 150;
> +
> +/* The number of pages that can be allocated globally for backing storage. */
> +static atomic_long_t sgx_nr_available_backing_pages;
> +
> +/**
> + * sgx_charge_mem() - charge for a page used for backing storage
> + *
> + * Backing storage usage is capped by the sgx_nr_available_backing_pages.
> + * If the backing storage usage is over the overcommit limit,
> + * return an error.
> + *
> + * Return:
> + * 0:		The page requested does not exceed the limit
> + * -ENOMEM:	The page requested exceeds the overcommit limit
> + */
> +int sgx_charge_mem(void)
> +{
> +	if (!atomic_long_add_unless(&sgx_nr_available_backing_pages, -1, 0))
> +		return -ENOMEM;
> +
> +	return 0;
> +}
> +
> +/**
> + * sgx_uncharge_mem() - uncharge a page previously used for backing storage
> + *
> + * When backing storage is no longer in use, increment the
> + * sgx_nr_available_backing_pages counter.
> + */
> +void sgx_uncharge_mem(void)
> +{
> +	atomic_long_inc(&sgx_nr_available_backing_pages);
> +}
> +
>  /*
>   * Reset post-kexec EPC pages to the uninitialized state. The pages are removed
>   * from the input list, and made available for the page allocator. SECS pages
> @@ -786,6 +825,8 @@ static bool __init sgx_page_cache_init(void)
>  	u64 pa, size;
>  	int nid;
>  	int i;
> +	u64 total_epc_bytes = 0;
> +	u64 available_backing_bytes;

Use reverse-christmas-tree declaration order here. This was required for
the original patch set, so I expect this to still hold.

>  
>  	sgx_numa_nodes = kmalloc_array(num_possible_nodes(), sizeof(*sgx_numa_nodes), GFP_KERNEL);
>  	if (!sgx_numa_nodes)
> @@ -830,6 +871,7 @@ static bool __init sgx_page_cache_init(void)
>  
>  		sgx_epc_sections[i].node =  &sgx_numa_nodes[nid];
>  		sgx_numa_nodes[nid].size += size;
> +		total_epc_bytes += size;
>  
>  		sgx_nr_epc_sections++;
>  	}
> @@ -839,6 +881,10 @@ static bool __init sgx_page_cache_init(void)
>  		return false;
>  	}
>  
> +	available_backing_bytes = total_epc_bytes * (sgx_overcommit_percent / 100);
> +

IMHO this empty line could be removed, and group these statements together.

> +	atomic_long_set(&sgx_nr_available_backing_pages, available_backing_bytes >> PAGE_SHIFT);
> +
>  	return true;
>  }
>  
> diff --git a/arch/x86/kernel/cpu/sgx/sgx.h b/arch/x86/kernel/cpu/sgx/sgx.h
> index 0f17def9fe6f..3507a9983fc1 100644
> --- a/arch/x86/kernel/cpu/sgx/sgx.h
> +++ b/arch/x86/kernel/cpu/sgx/sgx.h
> @@ -89,6 +89,8 @@ void sgx_free_epc_page(struct sgx_epc_page *page);
>  void sgx_mark_page_reclaimable(struct sgx_epc_page *page);
>  int sgx_unmark_page_reclaimable(struct sgx_epc_page *page);
>  struct sgx_epc_page *sgx_alloc_epc_page(void *owner, bool reclaim);
> +int sgx_charge_mem(void);
> +void sgx_uncharge_mem(void);
>  
>  #ifdef CONFIG_X86_SGX_KVM
>  int __init sgx_vepc_init(void);
> -- 
> 2.20.1
> 

Other than that, looks good to me.

BR,
Jarkko