mm: selftest to verify zero-filled pages are mapped to zeropage

JIRA: https://issues.redhat.com/browse/RHEL-88165
Conflicts:
  * tools/testing/selftests/mm/split_huge_page_test.c:
    This commit drops code related to `split_pmd_zero_pages()` from
    split_huge_page_test.c, as it is not needed for our backport.

    The goal is to apply only the vm_util.[ch] changes from the upstream
    patch series, while avoiding dependencies on upstream commit fc4d182
    ("mm: huge_memory: enable debugfs to split huge pages to any order"),
    which are not present in RHEL-9 and are unrelated to this backport effort.

This patch is a backport of the following upstream commit:
commit 391e869
Author: Alexander Zhu <alexlzhu@fb.com>
Date:   Fri Aug 30 11:03:37 2024 +0100

    mm: selftest to verify zero-filled pages are mapped to zeropage

    When a THP is split, any subpage that is zero-filled will be mapped to the
    shared zeropage, hence saving memory.  Add selftest to verify this by
    allocating zero-filled THP and comparing RssAnon before and after split.

    Link: https://lkml.kernel.org/r/20240830100438.3623486-4-usamaarif642@gmail.com
    Signed-off-by: Alexander Zhu <alexlzhu@fb.com>
    Signed-off-by: Usama Arif <usamaarif642@gmail.com>
    Acked-by: Rik van Riel <riel@surriel.com>
    Cc: Barry Song <baohua@kernel.org>
    Cc: David Hildenbrand <david@redhat.com>
    Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Jonathan Corbet <corbet@lwn.net>
    Cc: Kairui Song <ryncsn@gmail.com>
    Cc: Matthew Wilcox <willy@infradead.org>
    Cc: Mike Rapoport <rppt@kernel.org>
    Cc: Nico Pache <npache@redhat.com>
    Cc: Roman Gushchin <roman.gushchin@linux.dev>
    Cc: Ryan Roberts <ryan.roberts@arm.com>
    Cc: Shakeel Butt <shakeel.butt@linux.dev>
    Cc: Shuang Zhai <zhais@google.com>
    Cc: Yu Zhao <yuzhao@google.com>
    Cc: Shuang Zhai <szhai2@cs.rochester.edu>
    Cc: Hugh Dickins <hughd@google.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Li Wang <liwang@redhat.com>

Author: wangli5665

tools/testing/selftests/mm/vm_util.c

@@ -11,6 +11,7 @@
 
 #define PMD_SIZE_FILE_PATH "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size"
 #define SMAP_FILE_PATH "/proc/self/smaps"
+#define STATUS_FILE_PATH "/proc/self/status"
 #define MAX_LINE_LENGTH 500
 
 unsigned int __page_size;
@@ -97,6 +98,27 @@ uint64_t read_pmd_pagesize(void)
 	return strtoul(buf, NULL, 10);
 }
 
+unsigned long rss_anon(void)
+{
+	unsigned long rss_anon = 0;
+	FILE *fp;
+	char buffer[MAX_LINE_LENGTH];
+
+	fp = fopen(STATUS_FILE_PATH, "r");
+	if (!fp)
+		ksft_exit_fail_msg("%s: Failed to open file %s\n", __func__, STATUS_FILE_PATH);
+
+	if (!check_for_pattern(fp, "RssAnon:", buffer, sizeof(buffer)))
+		goto err_out;
+
+	if (sscanf(buffer, "RssAnon:%10lu kB", &rss_anon) != 1)
+		ksft_exit_fail_msg("Reading status error\n");
+
+err_out:
+	fclose(fp);
+	return rss_anon;
+}
+
 bool __check_huge(void *addr, char *pattern, int nr_hpages,
 		  uint64_t hpage_size)
 {

tools/testing/selftests/mm/vm_util.h

@@ -39,6 +39,7 @@ unsigned long pagemap_get_pfn(int fd, char *start);
 void clear_softdirty(void);
 bool check_for_pattern(FILE *fp, const char *pattern, char *buf, size_t len);
 uint64_t read_pmd_pagesize(void);
+unsigned long rss_anon(void);
 bool check_huge_anon(void *addr, int nr_hpages, uint64_t hpage_size);
 bool check_huge_file(void *addr, int nr_hpages, uint64_t hpage_size);
 bool check_huge_shmem(void *addr, int nr_hpages, uint64_t hpage_size);