[PATCH 1/5] x86/mm/pat: introcude cpa_lock() and cpa_unlock()

[Mike Rapoport (Microsoft)]

The splitting and merging of kernel page table mappings between small and
large is protected by cpa_lock.

Commit 5fce67641a3e ("x86/mm/pat: Don't gate cpa_lock on
debug_pagealloc_enabled()") disabled gatig of cpa_lock on
debug_pagealloc_enabled() to simplify the code presuming that skipping
the lock when debug_pagealloc_enabled() was an optimization.

However Lorenzo Stoakes notes that:

  __kernel_map_pages() can be called from irq context:

  < GFP_ATOMIC context >
  kfree() or whatever
  -> ...
  -> __free_pages_prepare()
  -> debug_pagealloc_unmap_pages()
  -> __kernel_map_pages()
  -> __change_page_attr_set_clr()
  -> cpa_lock spins [irqs off]

  So you're spin locking in irq context here, which is probably not a
  good idea.

It would be possible to unconditionally use spin_lock_irqsave() and
spin_unlock_irqrestore() but that would complicate locking rules even
more.

Restore gating of cpa_lock of debug_pagealloc_enabled(), but instead of
putting the open-coded condition

	if (debug_pagealloc_enabled())

before every lock and unlock operation, wrap the condition and the
locking operation into cpa_lock() and cpa_unlock() helpers.

Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
---
 arch/x86/mm/pat/set_memory.c | 33 +++++++++++++++++++++++++--------
 1 file changed, 25 insertions(+), 8 deletions(-)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index b1e780a465b5..4c8922695fd3 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -65,8 +65,25 @@ static const int cpa_warn_level = CPA_PROTECT;
  * Serialize cpa() using cpa_lock so that we don't allow any other cpu, with
  * stale large tlb entries, to change the page attribute in parallel to some
  * other cpu splitting a large page entry along with changing the attribute.
+ *
+ * When debug_pagealloc_enabled(), page attributes could be changed in atomic
+ * context that would warrant disabling IRQs. But since debug_pagealloc always
+ * uses 4k pages in the direct map there are no races for splits and collapses
+ * and locking can be just skipped altogether.
  */
-static DEFINE_SPINLOCK(cpa_lock);
+static DEFINE_SPINLOCK(_cpa_lock);
+
+static inline void cpa_lock(void)
+{
+	if (!debug_pagealloc_enabled())
+		spin_lock(&_cpa_lock);
+}
+
+static inline void cpa_unlock(void)
+{
+	if (!debug_pagealloc_enabled())
+		spin_unlock(&_cpa_lock);
+}
 
 #define CPA_FLUSHTLB 1
 #define CPA_ARRAY 2
@@ -417,7 +434,7 @@ static void cpa_collapse_large_pages(struct cpa_data *cpa)
 	int collapsed = 0;
 	int i;
 
-	spin_lock(&cpa_lock);
+	cpa_lock();
 
 	if (cpa->flags & (CPA_PAGES_ARRAY | CPA_ARRAY)) {
 		for (i = 0; i < cpa->numpages; i++)
@@ -433,7 +450,7 @@ static void cpa_collapse_large_pages(struct cpa_data *cpa)
 	}
 
 	if (!collapsed) {
-		spin_unlock(&cpa_lock);
+		cpa_unlock();
 		return;
 	}
 
@@ -444,7 +461,7 @@ static void cpa_collapse_large_pages(struct cpa_data *cpa)
 		pagetable_free(ptdesc);
 	}
 
-	spin_unlock(&cpa_lock);
+	cpa_unlock();
 }
 
 static void cpa_flush(struct cpa_data *cpa, int cache)
@@ -1239,9 +1256,9 @@ static int split_large_page(struct cpa_data *cpa, pte_t *kpte,
 {
 	struct ptdesc *ptdesc;
 
-	spin_unlock(&cpa_lock);
+	cpa_unlock();
 	ptdesc = pagetable_alloc(GFP_KERNEL, 0);
-	spin_lock(&cpa_lock);
+	cpa_lock();
 	if (!ptdesc)
 		return -ENOMEM;
 
@@ -2025,9 +2042,9 @@ static int __change_page_attr_set_clr(struct cpa_data *cpa, int primary)
 		if (cpa->flags & (CPA_ARRAY | CPA_PAGES_ARRAY))
 			cpa->numpages = 1;
 
-		spin_lock(&cpa_lock);
+		cpa_lock();
 		ret = __change_page_attr(cpa, primary);
-		spin_unlock(&cpa_lock);
+		cpa_unlock();
 		if (ret)
 			goto out;
 

-- 
2.53.0