mm: don't skip swap entry even if zap_details specified

Patch series "mm: Rework zap ptes on swap entries", v5.

Patch 1 should fix a long standing bug for zap_pte_range() on
zap_details usage.  The risk is we could have some swap entries skipped
while we should have zapped them.

Migration entries are not the major concern because file backed memory
always zap in the pattern that "first time without page lock, then
re-zap with page lock" hence the 2nd zap will always make sure all
migration entries are already recovered.

However there can be issues with real swap entries got skipped
errornoously.  There's a reproducer provided in commit message of patch
1 for that.

Patch 2-4 are cleanups that are based on patch 1.  After the whole
patchset applied, we should have a very clean view of zap_pte_range().

Only patch 1 needs to be backported to stable if necessary.

This patch (of 4):

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.

[[email protected]: comment tweaks]
  Link: https://lkml.kernel.org/r/[email protected]

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: Peter Xu <[email protected]>
Reviewed-by: John Hubbard <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/memory.c

@@ -1313,18 +1313,33 @@ 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.
  */
 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);