linux-kernel - [PATCH] mm/mapping_dirty_helpers: Update huge page-table entry callbacks

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Message-Id: <20200203154305.15045-1-thomas_os@shipmail.org>
Date:   Mon,  3 Feb 2020 16:43:05 +0100
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Thomas Hellstrom <thellstrom@...are.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Price <steven.price@....com>
Subject: [PATCH] mm/mapping_dirty_helpers: Update huge page-table entry callbacks

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

Following the update of pagewalk code
commit a07984d48146 ("mm: pagewalk: add p4d_entry() and pgd_entry()")
we can modify the mapping_dirty_helpers' huge page-table entry callbacks
to avoid splitting when a huge pud or -pmd is encountered.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Thomas Hellstrom <thellstrom@...are.com>
Reviewed-by: Steven Price <steven.price@....com>
---
v2: Added R-B from Steven.
---
 mm/mapping_dirty_helpers.c | 42 ++++++++++++++++++++++++++++++++++----
 1 file changed, 38 insertions(+), 4 deletions(-)

diff --git a/mm/mapping_dirty_helpers.c b/mm/mapping_dirty_helpers.c
index 71070dda9643..2c7d03675903 100644
--- a/mm/mapping_dirty_helpers.c
+++ b/mm/mapping_dirty_helpers.c
@@ -111,26 +111,60 @@ static int clean_record_pte(pte_t *pte, unsigned long addr,
 	return 0;
 }
 
-/* wp_clean_pmd_entry - The pagewalk pmd callback. */
+/*
+ * wp_clean_pmd_entry - The pagewalk pmd callback.
+ *
+ * Dirty-tracking should take place on the PTE level, so
+ * WARN() if encountering a dirty huge pmd.
+ * Furthermore, never split huge pmds, since that currently
+ * causes dirty info loss. The pagefault handler should do
+ * that if needed.
+ */
 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_unstable(&pmdval))
+		return 0;
+
+	if (pmd_none(pmdval)) {
+		walk->action = ACTION_AGAIN;
+		return 0;
+	}
+
+	/* Huge pmd, present or migrated */
+	walk->action = ACTION_CONTINUE;
 	if (pmd_trans_huge(pmdval) || pmd_devmap(pmdval))
 		WARN_ON(pmd_write(pmdval) || pmd_dirty(pmdval));
 
 	return 0;
 }
 
-/* wp_clean_pud_entry - The pagewalk pud callback. */
+/*
+ * wp_clean_pud_entry - The pagewalk pud callback.
+ *
+ * Dirty-tracking should take place on the PTE level, so
+ * WARN() if encountering a dirty huge puds.
+ * Furthermore, never split huge puds, since that currently
+ * causes dirty info loss. The pagefault handler should do
+ * that if needed.
+ */
 static int wp_clean_pud_entry(pud_t *pud, unsigned long addr, unsigned long end,
 			      struct mm_walk *walk)
 {
-	/* Dirty-tracking should be handled on the pte level */
 	pud_t pudval = READ_ONCE(*pud);
 
+	if (!pud_trans_unstable(&pudval))
+		return 0;
+
+	if (pud_none(pudval)) {
+		walk->action = ACTION_AGAIN;
+		return 0;
+	}
+
+	/* Huge pud */
+	walk->action = ACTION_CONTINUE;
 	if (pud_trans_huge(pudval) || pud_devmap(pudval))
 		WARN_ON(pud_write(pudval) || pud_dirty(pudval));
 
-- 
2.21.1