linux-kernel - Re: [PATCH] mm: move rmap of mTHP upon CoW reuse

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Message-ID: <427kxhljdcrn7thput727j6vpqvxtalistn6yoq6ykdpbe5435@sn7a4rh7zcdo>
Date: Thu, 25 Sep 2025 10:31:54 +0100
From: Kiryl Shutsemau <kirill@...temov.name>
To: Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, david@...hat.com, 
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz, rppt@...nel.org, 
	surenb@...gle.com, mhocko@...e.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: move rmap of mTHP upon CoW reuse

On Thu, Sep 25, 2025 at 02:24:29PM +0530, Dev Jain wrote:
> At wp-fault time, when we find that a folio is exclusively mapped, we move
> folio->mapping to the faulting VMA's anon_vma, so that rmap overhead
> reduces. This is currently done for small folios (base pages) and
> PMD-mapped THPs. Do this for mTHP too.
> 
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---
> mm-selftests pass.
> 
>  mm/memory.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 7e32eb79ba99..ec04d2cec6b1 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4014,6 +4014,11 @@ static bool __wp_can_reuse_large_anon_folio(struct folio *folio,
>  	 * an additional folio reference and never ended up here.
>  	 */
>  	exclusive = true;
> +
> +	if (folio_trylock(folio)) {
> +		folio_move_anon_rmap(folio, vma);
> +		folio_unlock(folio);
> +	}

Maybe take the folio lock earlier in wp_can_reuse_anon_folio() to cover
large folio handling too and avoid trylock here.

Something like this (untest):

diff --git a/mm/memory.c b/mm/memory.c
index 812a7d9f6531..d95cf670b6a8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3843,6 +3843,7 @@ static bool __wp_can_reuse_large_anon_folio(struct folio *folio,
 	 * an additional folio reference and never ended up here.
 	 */
 	exclusive = true;
+	folio_move_anon_rmap(folio, vma);
 unlock:
 	folio_unlock_large_mapcount(folio);
 	return exclusive;
@@ -3858,8 +3859,15 @@ static bool __wp_can_reuse_large_anon_folio(struct folio *folio,
 static bool wp_can_reuse_anon_folio(struct folio *folio,
 				    struct vm_area_struct *vma)
 {
-	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && folio_test_large(folio))
-		return __wp_can_reuse_large_anon_folio(folio, vma);
+	bool exclusive = false;
+
+	if (!folio_trylock(folio))
+		return false;
+
+	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && folio_test_large(folio)) {
+		exclusive = __wp_can_reuse_large_anon_folio(folio, vma);
+		goto unlock;
+	}
 
 	/*
 	 * We have to verify under folio lock: these early checks are
@@ -3869,7 +3877,8 @@ static bool wp_can_reuse_anon_folio(struct folio *folio,
 	 * KSM doesn't necessarily raise the folio refcount.
 	 */
 	if (folio_test_ksm(folio) || folio_ref_count(folio) > 3)
-		return false;
+		goto unlock;
+
 	if (!folio_test_lru(folio))
 		/*
 		 * We cannot easily detect+handle references from
@@ -3877,23 +3886,23 @@ static bool wp_can_reuse_anon_folio(struct folio *folio,
 		 */
 		lru_add_drain();
 	if (folio_ref_count(folio) > 1 + folio_test_swapcache(folio))
-		return false;
-	if (!folio_trylock(folio))
-		return false;
+		goto unlock;
+
 	if (folio_test_swapcache(folio))
 		folio_free_swap(folio);
-	if (folio_test_ksm(folio) || folio_ref_count(folio) != 1) {
-		folio_unlock(folio);
-		return false;
-	}
+	if (folio_test_ksm(folio) || folio_ref_count(folio) != 1)
+		goto unlock;
+
 	/*
 	 * Ok, we've got the only folio reference from our mapping
 	 * and the folio is locked, it's dark out, and we're wearing
 	 * sunglasses. Hit it.
 	 */
 	folio_move_anon_rmap(folio, vma);
+	exclusive = true;
+unlock:
 	folio_unlock(folio);
-	return true;
+	return ret;
 }
 
 /*
-- 
  Kiryl Shutsemau / Kirill A. Shutemov