[PATCH 6.6.y] mm/damon/paddr: drop last same folio access check reuse optimization

[SJ Park <sj@kernel.org>]

It can race when multiple kdamonds are being used.  The problem from the
race is doubtful, but the gain from the optimization is also doubtful.
Simply drop the optimization in favor of code simplicity.

The user impact is doubtfully trivial.  After all, this kind of
interference can happen only by intentional user setup.  Even if it
happens, it will be rare, and the consequence is degradation of the
best-effort monitoring results.  No critical consequences like kernel
panic or memory corruption happen.

The race was discovered [1] by Sashiko.

Link: https://lore.kernel.org/20260715031002.108504-5-sj@kernel.org
Link: https://lore.kernel.org/20260621204050.10993-1-sj@kernel.org [1]
Fixes: a28397beb55b ("mm/damon: implement primitives for physical address space monitoring")
Signed-off-by: SJ Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 5.16.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit f23f0aa62b2f32c2b12f95959fc4603ef81678b2)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/paddr.c | 21 ++++-----------------
 1 file changed, 4 insertions(+), 17 deletions(-)

diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
index 909db25efb35e..fe481ae64f37e 100644
--- a/mm/damon/paddr.c
+++ b/mm/damon/paddr.c
@@ -111,7 +111,7 @@ static bool __damon_pa_young(struct folio *folio, struct vm_area_struct *vma,
 	return *accessed == false;
 }
 
-static bool damon_pa_young(unsigned long paddr, unsigned long *folio_sz)
+static bool damon_pa_young(unsigned long paddr)
 {
 	struct folio *folio = damon_get_folio(PHYS_PFN(paddr));
 	bool accessed = false;
@@ -143,30 +143,17 @@ static bool damon_pa_young(unsigned long paddr, unsigned long *folio_sz)
 		folio_unlock(folio);
 
 out:
-	*folio_sz = folio_size(folio);
 	folio_put(folio);
 	return accessed;
 }
 
 static void __damon_pa_check_access(struct damon_region *r)
 {
-	static unsigned long last_addr;
-	static unsigned long last_folio_sz = PAGE_SIZE;
-	static bool last_accessed;
-
-	/* If the region is in the last checked page, reuse the result */
-	if (ALIGN_DOWN(last_addr, last_folio_sz) ==
-				ALIGN_DOWN(r->sampling_addr, last_folio_sz)) {
-		if (last_accessed)
-			r->nr_accesses++;
-		return;
-	}
+	bool accessed;
 
-	last_accessed = damon_pa_young(r->sampling_addr, &last_folio_sz);
-	if (last_accessed)
+	accessed = damon_pa_young(r->sampling_addr);
+	if (accessed)
 		r->nr_accesses++;
-
-	last_addr = r->sampling_addr;
 }
 
 static unsigned int damon_pa_check_accesses(struct damon_ctx *ctx)
-- 
2.47.3