linux-kernel - [PATCH 3/3] mm: Introduce ZAP_FLAG_SKIP

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Message-Id: <20201208025022.8234-4-peterx@redhat.com>
Date:   Mon,  7 Dec 2020 21:50:22 -0500
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     Andrea Arcangeli <aarcange@...hat.com>, peterx@...hat.com,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 3/3] mm: Introduce ZAP_FLAG_SKIP_SWAP

Firstly, the comment in zap_pte_range() is misleading because it checks against
details rather than check_mappings, so it's against what the code did.

Meanwhile, it's confusing too on not explaining why passing in the details
pointer would mean to skip all swap entries.  New user of zap_details could
very possibly miss this fact if they don't read deep until zap_pte_range()
because there's no comment at zap_details talking about it at all, so swap
entries could be errornously skipped without being noticed.

This partly reverts 3e8715fdc03e ("mm: drop zap_details::check_swap_entries"),
but introduce ZAP_FLAG_SKIP_SWAP flag, which means the opposite of previous
"details" parameter: the caller should explicitly set this to skip swap
entries, otherwise swap entries will always be considered (which is still the
major case here).

Cc: Kirill A. Shutemov <kirill@...temov.name>
Signed-off-by: Peter Xu <peterx@...hat.com>
---
 include/linux/mm.h | 12 ++++++++++++
 mm/memory.c        |  8 +++++---
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7ed4352ec84f..16631ee5eb9d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1622,6 +1622,8 @@ extern void user_shm_unlock(size_t, struct user_struct *);
 
 /* Whether to check page->mapping when zapping */
 #define  ZAP_FLAG_CHECK_MAPPING             BIT(0)
+/* Whether to skip zapping swap entries */
+#define  ZAP_FLAG_SKIP_SWAP                 BIT(1)
 
 /*
  * Parameter block passed down to zap_pte_range in exceptional cases.
@@ -1644,6 +1646,16 @@ zap_check_mapping_skip(struct zap_details *details, struct page *page)
 	return details->zap_mapping != page_rmapping(page);
 }
 
+/* Return true if skip swap entries, false otherwise */
+static inline bool
+zap_skip_swap(struct zap_details *details)
+{
+	if (!details)
+		return false;
+
+	return details->zap_flags & ZAP_FLAG_SKIP_SWAP;
+}
+
 struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
 			     pte_t pte);
 struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
diff --git a/mm/memory.c b/mm/memory.c
index 20a8ba05c334..c9945f3c374d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1262,8 +1262,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 			continue;
 		}
 
-		/* If details->check_mapping, we leave swap entries. */
-		if (unlikely(details))
+		if (unlikely(zap_skip_swap(details)))
 			continue;
 
 		if (!non_swap_entry(entry))
@@ -3159,7 +3158,10 @@ void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
 		pgoff_t nr, bool even_cows)
 {
 	pgoff_t	first_index = start, last_index = start + nr - 1;
-	struct zap_details details = { .zap_mapping = mapping };
+	struct zap_details details = {
+		.zap_mapping = mapping,
+		.zap_flags = ZAP_FLAG_SKIP_SWAP,
+	};
 
 	if (!even_cows)
 		details.zap_flags |= ZAP_FLAG_CHECK_MAPPING;
-- 
2.26.2