[PATCH RFC 01/18] x86/mm/pat: Don't gate cpa_lock on debug_pagealloc_enabled()

[Mike Rapoport (Microsoft)]

The splitting and merging of kernel page table mappings between small and
large is protected by cpa_lock. The merging is relatively new but the
splitting is ancient.

The splitting has a locking optimization: since DEBUG_PAGEALLOC forces all
mappings to 4k, there are no large pages to split. So the code that *might*
cause a split can just skip the locking (and a few other things).

This is entertaining, but it adds complexity and makes for weird locking
rules. Plus it's all for a debugging feature which makes the kernel super
slow in the first place. Optimizing something which is already super slow
and not used in production is not the best way to spend our complexity
budget.

Stop gating cpa_lock on debug_pagealloc_enabled() to simplify the code
and the locking rules.

[ dhansen: flesh out changelog ]

Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Link: https://patch.msgid.link/20260715144519.934289-1-rppt@xxxxxxxxxx
Link: https://lore.kernel.org/all/aab44f08-89f8-47fe-bee4-0ab6b25968c6@xxxxxxxxx/
---
 arch/x86/mm/pat/set_memory.c | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index d023a40a1e03..e8316f5ffa8a 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -62,10 +62,9 @@ enum cpa_warn {
 static const int cpa_warn_level = CPA_PROTECT;
 
 /*
- * Serialize cpa() (for !DEBUG_PAGEALLOC which uses large identity mappings)
- * using cpa_lock. So that we don't allow any other cpu, with stale large tlb
- * entries change the page attribute in parallel to some other cpu
- * splitting a large page entry along with changing the attribute.
+ * 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.
  */
 static DEFINE_SPINLOCK(cpa_lock);
 
@@ -1235,11 +1234,9 @@ static int split_large_page(struct cpa_data *cpa, pte_t *kpte,
 {
 	struct ptdesc *ptdesc;
 
-	if (!debug_pagealloc_enabled())
-		spin_unlock(&cpa_lock);
+	spin_unlock(&cpa_lock);
 	ptdesc = pagetable_alloc(GFP_KERNEL, 0);
-	if (!debug_pagealloc_enabled())
-		spin_lock(&cpa_lock);
+	spin_lock(&cpa_lock);
 	if (!ptdesc)
 		return -ENOMEM;
 
@@ -2023,11 +2020,9 @@ static int __change_page_attr_set_clr(struct cpa_data *cpa, int primary)
 		if (cpa->flags & (CPA_ARRAY | CPA_PAGES_ARRAY))
 			cpa->numpages = 1;
 
-		if (!debug_pagealloc_enabled())
-			spin_lock(&cpa_lock);
+		spin_lock(&cpa_lock);
 		ret = __change_page_attr(cpa, primary);
-		if (!debug_pagealloc_enabled())
-			spin_unlock(&cpa_lock);
+		spin_unlock(&cpa_lock);
 		if (ret)
 			goto out;
 

-- 
2.53.0