Date:   Thu, 17 Feb 2022 14:07:43 +0800
From:   Peter Xu <peterx@...hat.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Alistair Popple <apopple@...dia.com>,
        Matthew Wilcox <willy@...radead.org>, peterx@...hat.com,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: [PATCH v5 1/4] mm: Don't skip swap entry even if zap_details specified

The "details" pointer shouldn't be the token to decide whether we should
skip swap entries.

For example, when the callers specified details->zap_mapping==NULL, it
means the user wants to zap all the pages (including COWed pages), then we
need to look into swap entries because there can be private COWed pages
that was swapped out.

Skipping some swap entries when details is non-NULL may lead to wrongly
leaving some of the swap entries while we should have zapped them.

A reproducer of the problem:

===8<===
        #define _GNU_SOURCE         /* See feature_test_macros(7) */
        #include <stdio.h>
        #include <assert.h>
        #include <unistd.h>
        #include <sys/mman.h>
        #include <sys/types.h>

        int page_size;
        int shmem_fd;
        char *buffer;

        void main(void)
        {
                int ret;
                char val;

                page_size = getpagesize();
                shmem_fd = memfd_create("test", 0);
                assert(shmem_fd >= 0);

                ret = ftruncate(shmem_fd, page_size * 2);
                assert(ret == 0);

                buffer = mmap(NULL, page_size * 2, PROT_READ | PROT_WRITE,
                                MAP_PRIVATE, shmem_fd, 0);
                assert(buffer != MAP_FAILED);

                /* Write private page, swap it out */
                buffer[page_size] = 1;
                madvise(buffer, page_size * 2, MADV_PAGEOUT);

                /* This should drop private buffer[page_size] already */
                ret = ftruncate(shmem_fd, page_size);
                assert(ret == 0);
                /* Recover the size */
                ret = ftruncate(shmem_fd, page_size * 2);
                assert(ret == 0);

                /* Re-read the data, it should be all zero */
                val = buffer[page_size];
                if (val == 0)
                        printf("Good\n");
                else
                        printf("BUG\n");
        }
===8<===

We don't need to touch up the pmd path, because pmd never had a issue with
swap entries.  For example, shmem pmd migration will always be split into
pte level, and same to swapping on anonymous.

Add another helper should_zap_cows() so that we can also check whether we
should zap private mappings when there's no page pointer specified.

This patch drops that trick, so we handle swap ptes coherently.  Meanwhile
we should do the same check upon migration entry, hwpoison entry and
genuine swap entries too.

To be explicit, we should still remember to keep the private entries if
even_cows==false, and always zap them when even_cows==true.

The issue seems to exist starting from the initial commit of git.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reviewed-by: John Hubbard <jhubbard@...dia.com>
Signed-off-by: Peter Xu <peterx@...hat.com>
---
 mm/memory.c | 40 +++++++++++++++++++++++++++++++---------
 1 file changed, 31 insertions(+), 9 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index c125c4969913..533da5d6c32c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1313,6 +1313,17 @@ struct zap_details {
 	struct folio *single_folio;	/* Locked folio to be unmapped */
 };
 
+/* Whether we should zap all COWed (private) pages too */
+static inline bool should_zap_cows(struct zap_details *details)
+{
+	/* By default, zap all pages */
+	if (!details)
+		return true;
+
+	/* Or, we zap COWed pages only if the caller wants to */
+	return !details->zap_mapping;
+}
+
 /*
  * We set details->zap_mapping when we want to unmap shared but keep private
  * pages. Return true if skip zapping this page, false otherwise.
@@ -1320,11 +1331,15 @@ struct zap_details {
 static inline bool
 zap_skip_check_mapping(struct zap_details *details, struct page *page)
 {
-	if (!details || !page)
+	/* If we can make a decision without *page.. */
+	if (should_zap_cows(details))
+		return false;
+
+	/* E.g. the caller passes NULL for the case of a zero page */
+	if (!page)
 		return false;
 
-	return details->zap_mapping &&
-		(details->zap_mapping != page_rmapping(page));
+	return details->zap_mapping != page_rmapping(page);
 }
 
 static unsigned long zap_pte_range(struct mmu_gather *tlb,
@@ -1405,17 +1420,24 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 			continue;
 		}
 
-		/* If details->check_mapping, we leave swap entries. */
-		if (unlikely(details))
-			continue;
-
-		if (!non_swap_entry(entry))
+		if (!non_swap_entry(entry)) {
+			/* Genuine swap entry, hence a private anon page */
+			if (!should_zap_cows(details))
+				continue;
 			rss[MM_SWAPENTS]--;
-		else if (is_migration_entry(entry)) {
+		} else if (is_migration_entry(entry)) {
 			struct page *page;
 
 			page = pfn_swap_entry_to_page(entry);
+			if (zap_skip_check_mapping(details, page))
+				continue;
 			rss[mm_counter(page)]--;
+		} else if (is_hwpoison_entry(entry)) {
+			if (!should_zap_cows(details))
+				continue;
+		} else {
+			/* We should have covered all the swap entry types */
+			WARN_ON_ONCE(1);
 		}
 		if (unlikely(!free_swap_and_cache(entry)))
 			print_bad_pte(vma, addr, ptent, NULL);
-- 
2.32.0

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