migrate: fix syscall move_pages() return value for failure

Patch series "migrate_pages(): fix several bugs in error path", v3.

During review the code of migrate_pages() and build a test program for
it.  Several bugs in error path are identified and fixed in this
series.

Most patches are tested via

- Apply error-inject.patch in Linux kernel
- Compile test-migrate.c (with -lnuma)
- Test with test-migrate.sh

error-inject.patch, test-migrate.c, and test-migrate.sh are as below.
It turns out that error injection is an important tool to fix bugs in
error path.


This patch (of 8):

The return value of move_pages() syscall is incorrect when counting
the remaining pages to be migrated.  For example, for the following
test program,

"
 #define _GNU_SOURCE

 #include <stdbool.h>
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
 #include <errno.h>

 #include <fcntl.h>
 #include <sys/uio.h>
 #include <sys/mman.h>
 #include <sys/types.h>
 #include <unistd.h>
 #include <numaif.h>
 #include <numa.h>

 #ifndef MADV_FREE
 #define MADV_FREE	8		/* free pages only if memory pressure */
 #endif

 #define ONE_MB		(1024 * 1024)
 #define MAP_SIZE	(16 * ONE_MB)
 #define THP_SIZE	(2 * ONE_MB)
 #define THP_MASK	(THP_SIZE - 1)

 #define ERR_EXIT_ON(cond, msg)					\
	 do {							\
		 int __cond_in_macro = (cond);			\
		 if (__cond_in_macro)				\
			 error_exit(__cond_in_macro, (msg));	\
	 } while (0)

 void error_msg(int ret, int nr, int *status, const char *msg)
 {
	 int i;

	 fprintf(stderr, "Error: %s, ret : %d, error: %s\n",
		 msg, ret, strerror(errno));

	 if (!nr)
		 return;
	 fprintf(stderr, "status: ");
	 for (i = 0; i < nr; i++)
		 fprintf(stderr, "%d ", status[i]);
	 fprintf(stderr, "\n");
 }

 void error_exit(int ret, const char *msg)
 {
	 error_msg(ret, 0, NULL, msg);
	 exit(1);
 }

 int page_size;

 bool do_vmsplice;
 bool do_thp;

 static int pipe_fds[2];
 void *addr;
 char *pn;
 char *pn1;
 void *pages[2];
 int status[2];

 void prepare()
 {
	 int ret;
	 struct iovec iov;

	 if (addr) {
		 munmap(addr, MAP_SIZE);
		 close(pipe_fds[0]);
		 close(pipe_fds[1]);
	 }

	 ret = pipe(pipe_fds);
	 ERR_EXIT_ON(ret, "pipe");

	 addr = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE,
		     MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
	 ERR_EXIT_ON(addr == MAP_FAILED, "mmap");
	 if (do_thp) {
		 ret = madvise(addr, MAP_SIZE, MADV_HUGEPAGE);
		 ERR_EXIT_ON(ret, "advise hugepage");
	 }

	 pn = (char *)(((unsigned long)addr + THP_SIZE) & ~THP_MASK);
	 pn1 = pn + THP_SIZE;
	 pages[0] = pn;
	 pages[1] = pn1;
	 *pn = 1;

	 if (do_vmsplice) {
		 iov.iov_base = pn;
		 iov.iov_len = page_size;
		 ret = vmsplice(pipe_fds[1], &iov, 1, 0);
		 ERR_EXIT_ON(ret < 0, "vmsplice");
	 }

	 status[0] = status[1] = 1024;
 }

 void test_migrate()
 {
	 int ret;
	 int nodes[2] = { 1, 1 };
	 pid_t pid = getpid();

	 prepare();
	 ret = move_pages(pid, 1, pages, nodes, status, MPOL_MF_MOVE_ALL);
	 error_msg(ret, 1, status, "move 1 page");

	 prepare();
	 ret = move_pages(pid, 2, pages, nodes, status, MPOL_MF_MOVE_ALL);
	 error_msg(ret, 2, status, "move 2 pages, page 1 not mapped");

	 prepare();
	 *pn1 = 1;
	 ret = move_pages(pid, 2, pages, nodes, status, MPOL_MF_MOVE_ALL);
	 error_msg(ret, 2, status, "move 2 pages");

	 prepare();
	 *pn1 = 1;
	 nodes[1] = 0;
	 ret = move_pages(pid, 2, pages, nodes, status, MPOL_MF_MOVE_ALL);
	 error_msg(ret, 2, status, "move 2 pages, page 1 to node 0");
 }

 int main(int argc, char *argv[])
 {
	 numa_run_on_node(0);
	 page_size = getpagesize();

	 test_migrate();

	 fprintf(stderr, "\nMake page 0 cannot be migrated:\n");
	 do_vmsplice = true;
	 test_migrate();

	 fprintf(stderr, "\nTest THP:\n");
	 do_thp = true;
	 do_vmsplice = false;
	 test_migrate();

	 fprintf(stderr, "\nTHP: make page 0 cannot be migrated:\n");
	 do_vmsplice = true;
	 test_migrate();

	 return 0;
 }
"

The output of the current kernel is,

"
Error: move 1 page, ret : 0, error: Success
status: 1
Error: move 2 pages, page 1 not mapped, ret : 0, error: Success
status: 1 -14
Error: move 2 pages, ret : 0, error: Success
status: 1 1
Error: move 2 pages, page 1 to node 0, ret : 0, error: Success
status: 1 0

Make page 0 cannot be migrated:
Error: move 1 page, ret : 0, error: Success
status: 1024
Error: move 2 pages, page 1 not mapped, ret : 1, error: Success
status: 1024 -14
Error: move 2 pages, ret : 0, error: Success
status: 1024 1024
Error: move 2 pages, page 1 to node 0, ret : 1, error: Success
status: 1024 1024
"

While the expected output is,

"
Error: move 1 page, ret : 0, error: Success
status: 1
Error: move 2 pages, page 1 not mapped, ret : 0, error: Success
status: 1 -14
Error: move 2 pages, ret : 0, error: Success
status: 1 1
Error: move 2 pages, page 1 to node 0, ret : 0, error: Success
status: 1 0

Make page 0 cannot be migrated:
Error: move 1 page, ret : 1, error: Success
status: 1024
Error: move 2 pages, page 1 not mapped, ret : 1, error: Success
status: 1024 -14
Error: move 2 pages, ret : 1, error: Success
status: 1024 1024
Error: move 2 pages, page 1 to node 0, ret : 2, error: Success
status: 1024 1024
"

Fix this via correcting the remaining pages counting.  With the fix,
the output for the test program as above is expected.

Link: https://lkml.kernel.org/r/20220817081408.513338-1-ying.huang@intel.com
Link: https://lkml.kernel.org/r/20220817081408.513338-2-ying.huang@intel.com
Fixes: 5984fabb6e ("mm: move_pages: report the number of non-attempted pages")
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/migrate.c

@@ -1751,7 +1751,7 @@ static int move_pages_and_store_status(struct mm_struct *mm, int node,
 		 * well.
 		 */
 		if (err > 0)
-			err += nr_pages - i - 1;
+			err += nr_pages - i;
 		return err;
 	}
 	return store_status(status, start, node, i - start);
@@ -1837,8 +1837,12 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
 
 		err = move_pages_and_store_status(mm, current_node, &pagelist,
 				status, start, i, nr_pages);
-		if (err)
+		if (err) {
+			/* We have accounted for page i */
+			if (err > 0)
+				err--;
 			goto out;
+		}
 		current_node = NUMA_NO_NODE;
 	}
 out_flush: