Re: [PATCH v5 07/18] x86/sgx: Introduce RECLAIM_IN_PROGRESS state

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Sat Sep 23, 2023 at 6:06 AM EEST, Haitao Huang wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
>
> Add RECLAIM_IN_PROGRESS state to not rely on list_empty(&epc_page->list)
> to determine if an EPC page is selected as a reclaiming candidate.
>
> When a page is being reclaimed from the page pool (sgx_global_lru),
> there is an intermediate stage where a page may have been identified as
> a candidate for reclaiming, but has not yet been reclaimed.  Currently
> such pages are list_del_init()'d from the global LRU list, and stored in
> a an array on stack. To prevent another thread from dropping the same
> page in the middle of reclaiming, sgx_drop_epc_page() checks for
> list_empty(&epc_page->list).
>
> A later patch will replace the array on stack with a temporary list to
> store the candidate pages, so list_empty() should no longer be used for
> this purpose.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Co-developed-by: Kristen Carlson Accardi <kristen@linux.intel.com>
> Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
> Co-developed-by: Haitao Huang <haitao.huang@linux.intel.com>
> Signed-off-by: Haitao Huang <haitao.huang@linux.intel.com>
> Cc: Sean Christopherson <seanjc@google.com>
> ---
> V4:
> - Fixed some typos.
> - Revised commit message.
>
> V3:
> - Extend the sgx_epc_page_state enum introduced earlier to replace the
> flag based approach.
> ---
>  arch/x86/kernel/cpu/sgx/main.c | 21 ++++++++++-----------
>  arch/x86/kernel/cpu/sgx/sgx.h  | 16 ++++++++++++++++
>  2 files changed, 26 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index b26860399402..c1ae19a154d0 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -312,13 +312,15 @@ static void sgx_reclaim_pages(void)
>  		list_del_init(&epc_page->list);
>  		encl_page = epc_page->owner;
>  
> -		if (kref_get_unless_zero(&encl_page->encl->refcount) != 0)
> +		if (kref_get_unless_zero(&encl_page->encl->refcount) != 0) {
> +			sgx_epc_page_set_state(epc_page, SGX_EPC_PAGE_RECLAIM_IN_PROGRESS);
>  			chunk[cnt++] = epc_page;
> -		else
> +		} else {
>  			/* The owner is freeing the page. No need to add the
>  			 * page back to the list of reclaimable pages.
>  			 */
>  			sgx_epc_page_reset_state(epc_page);
> +		}
>  	}
>  	spin_unlock(&sgx_global_lru.lock);
>  
> @@ -528,16 +530,13 @@ void sgx_record_epc_page(struct sgx_epc_page *page, unsigned long flags)
>  int sgx_drop_epc_page(struct sgx_epc_page *page)
>  {
>  	spin_lock(&sgx_global_lru.lock);
> -	if (sgx_epc_page_reclaimable(page->flags)) {
> -		/* The page is being reclaimed. */
> -		if (list_empty(&page->list)) {
> -			spin_unlock(&sgx_global_lru.lock);
> -			return -EBUSY;
> -		}
> -
> -		list_del(&page->list);
> -		sgx_epc_page_reset_state(page);
> +	if (sgx_epc_page_reclaim_in_progress(page->flags)) {
> +		spin_unlock(&sgx_global_lru.lock);
> +		return -EBUSY;
>  	}
> +
> +	list_del(&page->list);
> +	sgx_epc_page_reset_state(page);
>  	spin_unlock(&sgx_global_lru.lock);
>  
>  	return 0;
> diff --git a/arch/x86/kernel/cpu/sgx/sgx.h b/arch/x86/kernel/cpu/sgx/sgx.h
> index 2faeb40b345f..764cec23f4e5 100644
> --- a/arch/x86/kernel/cpu/sgx/sgx.h
> +++ b/arch/x86/kernel/cpu/sgx/sgx.h
> @@ -40,6 +40,8 @@ enum sgx_epc_page_state {
>  
>  	/* Page is in use and tracked in a reclaimable LRU list
>  	 * Becomes NOT_TRACKED after sgx_drop_epc()
> +	 * Becomes RECLAIM_IN_PROGRESS in sgx_reclaim_pages() when identified
> +	 * for reclaiming
>  	 */
>  	SGX_EPC_PAGE_RECLAIMABLE = 2,
>  
> @@ -50,6 +52,14 @@ enum sgx_epc_page_state {
>  	 */
>  	SGX_EPC_PAGE_UNRECLAIMABLE = 3,
>  
> +	/* Page is being prepared for reclamation, tracked in a temporary
> +	 * isolated list by the reclaimer.
> +	 * Changes in sgx_reclaim_pages() back to RECLAIMABLE if preparation
> +	 * fails for any reason.
> +	 * Becomes NOT_TRACKED if reclaimed successfully in sgx_reclaim_pages()
> +	 * and immediately sgx_free_epc() is called to make it FREE.
> +	 */
> +	SGX_EPC_PAGE_RECLAIM_IN_PROGRESS = 4,
>  };
>  
>  #define SGX_EPC_PAGE_STATE_MASK GENMASK(2, 0)
> @@ -73,6 +83,12 @@ static inline void sgx_epc_page_set_state(struct sgx_epc_page *page, unsigned lo
>  	page->flags |= (flags & SGX_EPC_PAGE_STATE_MASK);
>  }
>  
> +static inline bool sgx_epc_page_reclaim_in_progress(unsigned long flags)
> +{
> +	return SGX_EPC_PAGE_RECLAIM_IN_PROGRESS == (flags &
> +						    SGX_EPC_PAGE_STATE_MASK);
> +}

	return SGX_EPC_PAGE_RECLAIM_IN_PROGRESS == (flags & SGX_EPC_PAGE_STATE_MASK);

> +
>  static inline bool sgx_epc_page_reclaimable(unsigned long flags)
>  {
>  	return SGX_EPC_PAGE_RECLAIMABLE == (flags & SGX_EPC_PAGE_STATE_MASK);
> -- 
> 2.25.1


BR, Jarkko