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

Jarkko Sakkinen
2023-09-25T17:13:12Z

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