[PATCH v3 08/28] x86/sgx: Introduce RECLAIM_IN_PROGRESS state

[Haitao Huang]

From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

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, 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(&page->list).

In future patches these pages need be list_move()'d into a temporary
list that is shared with multiple cgroup reclaimers. so list_empty()
should no longer be used for this purpose. Add a RECLAIM_IN_PROGRESS
state to explicitly designate such intermediate state of EPC in the
reclaiming process. Do not drop any page in this state in
sgx_drop_epc_page().

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
Signed-off-by: Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx>
Cc: Sean Christopherson <seanjc@xxxxxxxxxx>

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 02c358f10383..9eea9038758f 100644
--- a/arch/x86/kernel/cpu/sgx/main.c
+++ b/arch/x86/kernel/cpu/sgx/main.c
@@ -313,13 +313,15 @@ static void sgx_reclaim_pages(void)
 		list_del_init(&epc_page->list);
 		encl_page = epc_page->encl_page;
 
-		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);
 
@@ -531,16 +533,13 @@ 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 057905eba466..f26ed4c0d12f 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 reclaimation, 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)
@@ -82,6 +92,12 @@ 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);
+}
+
 static inline bool sgx_epc_page_reclaimable(unsigned long flags)
 {
 	return SGX_EPC_PAGE_RECLAIMABLE == (flags & SGX_EPC_PAGE_STATE_MASK);
-- 
2.25.1