linux-kernel - [RFC PATCH 4/4] mm: mapping_dirty_helpers: Handle huge pmds correctly

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Date:   Thu, 10 Oct 2019 15:40:58 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        torvalds@...ux-foundation.org, kirill@...temov.name
Cc:     Thomas Hellstrom <thellstrom@...are.com>
Subject: [RFC PATCH 4/4] mm: mapping_dirty_helpers: Handle huge pmds correctly

From: Thomas Hellstrom <thellstrom@...are.com>

We always do dirty tracking on the PTE level. This means that any huge
pmds we encounter should be read-only and not dirty: We can just skip
those. Write-enabled huge pmds should not exist. They should have been
split when made write-enabled. Warn and attempt to split them.

Signed-off-by: Thomas Hellstrom <thellstrom@...are.com>
---
 mm/mapping_dirty_helpers.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/mm/mapping_dirty_helpers.c b/mm/mapping_dirty_helpers.c
index 799b9154b48f..f61bb9de1530 100644
--- a/mm/mapping_dirty_helpers.c
+++ b/mm/mapping_dirty_helpers.c
@@ -115,11 +115,18 @@ static int clean_record_pte(pte_t *pte, unsigned long addr,
 static int wp_clean_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long end,
 			      struct mm_walk *walk)
 {
-	/* Dirty-tracking should be handled on the pte level */
 	pmd_t pmdval = pmd_read_atomic(pmd);
 
-	if (pmd_trans_huge(pmdval) || pmd_devmap(pmdval))
-		WARN_ON(pmd_write(pmdval) || pmd_dirty(pmdval));
+	/*
+	 * Dirty-tracking should be handled on the pte level, and write-
+	 * enabled huge PMDS should never have been created. Warn on those.
+	 * Read-only huge PMDS can't be dirty so we just skip them.
+	 */
+	if (pmd_trans_huge(pmdval) || pmd_devmap(pmdval)) {
+		if (WARN_ON(pmd_write(pmdval) || pmd_dirty(pmdval)))
+			return 0;
+		return PAGE_WALK_CONTINUE;
+	}
 
 	return 0;
 }
-- 
2.21.0