linux-kernel - [PATCH] mempolicy: Optimize queue_folios_pte

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Message-Id: <20250411081301.8533-1-dev.jain@arm.com>
Date: Fri, 11 Apr 2025 13:43:01 +0530
From: Dev Jain <dev.jain@....com>
To: akpm@...ux-foundation.org
Cc: ryan.roberts@....com,
	david@...hat.com,
	willy@...radead.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	hughd@...gle.com,
	vishal.moola@...il.com,
	yang@...amperecomputing.com,
	ziy@...dia.com,
	Dev Jain <dev.jain@....com>
Subject: [PATCH] mempolicy: Optimize queue_folios_pte_range by PTE batching

After the check for queue_folio_required(), the code only cares about the
folio in the for loop, i.e the PTEs are redundant. Therefore, optimize this
loop by skipping over a PTE batch mapping the same folio.

Signed-off-by: Dev Jain <dev.jain@....com>
---
Unfortunately I have only build tested this since my test environment is
broken.

 mm/mempolicy.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index b28a1e6ae096..b019524da8a2 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -573,6 +573,9 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
 	pte_t *pte, *mapped_pte;
 	pte_t ptent;
 	spinlock_t *ptl;
+	int max_nr;
+	const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
+	int nr = 1;
 
 	ptl = pmd_trans_huge_lock(pmd, vma);
 	if (ptl) {
@@ -586,7 +589,8 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
 		walk->action = ACTION_AGAIN;
 		return 0;
 	}
-	for (; addr != end; pte++, addr += PAGE_SIZE) {
+	for (; addr != end; pte += nr, addr += nr * PAGE_SIZE) {
+		nr = 1;
 		ptent = ptep_get(pte);
 		if (pte_none(ptent))
 			continue;
@@ -607,6 +611,11 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
 		if (!queue_folio_required(folio, qp))
 			continue;
 		if (folio_test_large(folio)) {
+			max_nr = (end - addr) >> PAGE_SHIFT;
+			if (max_nr != 1)
+				nr = folio_pte_batch(folio, addr, pte, ptent,
+						     max_nr, fpb_flags,
+						     NULL, NULL, NULL);
 			/*
 			 * A large folio can only be isolated from LRU once,
 			 * but may be mapped by many PTEs (and Copy-On-Write may
@@ -633,6 +642,7 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
 			qp->nr_failed++;
 			if (strictly_unmovable(flags))
 				break;
+			qp->nr_failed += nr - 1;
 		}
 	}
 	pte_unmap_unlock(mapped_pte, ptl);
-- 
2.30.2