[PATCH 6.18.y] mm/damon/paddr: drop last same folio access check reuse optimization

[PATCH 6.18.y] mm/damon/paddr: drop last same folio access check reuse optimization

[SJ Park]

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 | 20 ++++----------------
 1 file changed, 4 insertions(+), 16 deletions(-)

diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
index f28cfee8351d3..1854b4bb6bd29 100644
--- a/mm/damon/paddr.c
+++ b/mm/damon/paddr.c
@@ -67,7 +67,7 @@ static void damon_pa_prepare_access_checks(struct damon_ctx *ctx)
 	}
 }
 
-static bool damon_pa_young(phys_addr_t paddr, unsigned long *folio_sz)
+static bool damon_pa_young(phys_addr_t paddr)
 {
 	struct folio *folio = damon_get_folio(PHYS_PFN(paddr));
 	bool accessed;
@@ -76,7 +76,6 @@ static bool damon_pa_young(phys_addr_t paddr, unsigned long *folio_sz)
 		return false;
 
 	accessed = damon_folio_young(folio);
-	*folio_sz = folio_size(folio);
 	folio_put(folio);
 	return accessed;
 }
@@ -84,23 +83,12 @@ static bool damon_pa_young(phys_addr_t paddr, unsigned long *folio_sz)
 static void __damon_pa_check_access(struct damon_region *r,
 		struct damon_attrs *attrs, unsigned long addr_unit)
 {
-	static phys_addr_t last_addr;
-	static unsigned long last_folio_sz = PAGE_SIZE;
-	static bool last_accessed;
+	bool accessed;
 	phys_addr_t sampling_addr = damon_pa_phys_addr(
 			r->sampling_addr, addr_unit);
 
-	/* If the region is in the last checked page, reuse the result */
-	if (ALIGN_DOWN(last_addr, last_folio_sz) ==
-				ALIGN_DOWN(sampling_addr, last_folio_sz)) {
-		damon_update_region_access_rate(r, last_accessed, attrs);
-		return;
-	}
-
-	last_accessed = damon_pa_young(sampling_addr, &last_folio_sz);
-	damon_update_region_access_rate(r, last_accessed, attrs);
-
-	last_addr = sampling_addr;
+	accessed = damon_pa_young(sampling_addr);
+	damon_update_region_access_rate(r, accessed, attrs);
 }
 
 static unsigned int damon_pa_check_accesses(struct damon_ctx *ctx)
-- 
2.47.3

Re: [PATCH 6.18.y] mm/damon/paddr: drop last same folio access check reuse optimization

[sashiko-bot]

> 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>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260909053834.248582-1-sj@kernel.org?part=1