[PATCH] mm: use only pidfd for process_madvise syscall

[Minchan Kim]

Based on discussion[1], people didn't feel we need to support both
pid and pidfd for every new coming API[2] so this patch keeps only
pidfd. This patch also changes flags's type with "unsigned int".
So finally, the API is as follows,

      ssize_t process_madvise(int pidfd, const struct iovec *iovec,
      		unsigned long vlen, int advice, unsigned int flags);

    DESCRIPTION
      The process_madvise() system call is used to give advice or directions
      to the kernel about the address ranges from external process as well as
      local process. It provides the advice to address ranges of process
      described by iovec and vlen. The goal of such advice is to improve system
      or application performance.

      The pidfd selects the process referred to by the PID file descriptor
      specified in pidfd. (See pidofd_open(2) for further information)

      The pointer iovec points to an array of iovec structures, defined in
      <sys/uio.h> as:

        struct iovec {
            void *iov_base;         /* starting address */
            size_t iov_len;         /* number of bytes to be advised */
        };

      The iovec describes address ranges beginning at address(iov_base)
      and with size length of bytes(iov_len).

      The vlen represents the number of elements in iovec.

      The advice is indicated in the advice argument, which is one of the
      following at this moment if the target process specified by idtype and
      id is external.

        MADV_COLD
        MADV_PAGEOUT
        MADV_MERGEABLE
        MADV_UNMERGEABLE

      Permission to provide a hint to external process is governed by a
      ptrace access mode PTRACE_MODE_ATTACH_FSCREDS check; see ptrace(2).

      The process_madvise supports every advice madvise(2) has if target
      process is in same thread group with calling process so user could
      use process_madvise(2) to extend existing madvise(2) to support
      vector address ranges.

    RETURN VALUE
      On success, process_madvise() returns the number of bytes advised.
      This return value may be less than the total number of requested
      bytes, if an error occurred. The caller should check return value
      to determine whether a partial advice occurred.

[1] 20200509124817.xmrvsrq3mla6b76k@wittgenstein/">https://lore.kernel.org/linux-mm/20200509124817.xmrvsrq3mla6b76k@wittgenstein/
[2] https://lore.kernel.org/linux-mm/9d849087-3359-c4ab-fbec-859e8186c509@xxxxxxxxxxxxx/
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/madvise.c | 42 +++++++++++++-----------------------------
 1 file changed, 13 insertions(+), 29 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index d3fbbe52d230..35c9b220146a 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1229,8 +1229,8 @@ static int process_madvise_vec(struct task_struct *target_task,
 	return ret;
 }
 
-static ssize_t do_process_madvise(int which, pid_t upid, struct iov_iter *iter,
-				       int behavior, unsigned long flags)
+static ssize_t do_process_madvise(int pidfd, struct iov_iter *iter,
+				int behavior, unsigned int flags)
 {
 	ssize_t ret;
 	struct pid *pid;
@@ -1241,26 +1241,12 @@ static ssize_t do_process_madvise(int which, pid_t upid, struct iov_iter *iter,
 	if (flags != 0)
 		return -EINVAL;
 
-	switch (which) {
-	case P_PID:
-		if (upid <= 0)
-			return -EINVAL;
-
-		pid = find_get_pid(upid);
-		if (!pid)
-			return -ESRCH;
-		break;
-	case P_PIDFD:
-		if (upid < 0)
-			return -EINVAL;
-
-		pid = pidfd_get_pid(upid);
-		if (IS_ERR(pid))
-			return PTR_ERR(pid);
-		break;
-	default:
+	if (pidfd < 0)
 		return -EINVAL;
-	}
+
+	pid = pidfd_get_pid(pidfd);
+	if (IS_ERR(pid))
+		return PTR_ERR(pid);
 
 	task = get_pid_task(pid, PIDTYPE_PID);
 	if (!task) {
@@ -1292,9 +1278,8 @@ static ssize_t do_process_madvise(int which, pid_t upid, struct iov_iter *iter,
 	return ret;
 }
 
-SYSCALL_DEFINE6(process_madvise, int, which, pid_t, upid,
-		const struct iovec __user *, vec, unsigned long, vlen,
-		int, behavior, unsigned long, flags)
+SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
+		unsigned long, vlen, int, behavior, unsigned int, flags)
 {
 	ssize_t ret;
 	struct iovec iovstack[UIO_FASTIOV];
@@ -1303,19 +1288,18 @@ SYSCALL_DEFINE6(process_madvise, int, which, pid_t, upid,
 
 	ret = import_iovec(READ, vec, vlen, ARRAY_SIZE(iovstack), &iov, &iter);
 	if (ret >= 0) {
-		ret = do_process_madvise(which, upid, &iter, behavior, flags);
+		ret = do_process_madvise(pidfd, &iter, behavior, flags);
 		kfree(iov);
 	}
 	return ret;
 }
 
 #ifdef CONFIG_COMPAT
-COMPAT_SYSCALL_DEFINE6(process_madvise, compat_int_t, which,
-			compat_pid_t, upid,
+COMPAT_SYSCALL_DEFINE5(process_madvise, compat_int_t, pidfd,
 			const struct compat_iovec __user *, vec,
 			compat_ulong_t, vlen,
 			compat_int_t, behavior,
-			compat_ulong_t, flags)
+			compat_int_t, flags)
 
 {
 	ssize_t ret;
@@ -1326,7 +1310,7 @@ COMPAT_SYSCALL_DEFINE6(process_madvise, compat_int_t, which,
 	ret = compat_import_iovec(READ, vec, vlen, ARRAY_SIZE(iovstack),
 				&iov, &iter);
 	if (ret >= 0) {
-		ret = do_process_madvise(which, upid, &iter, behavior, flags);
+		ret = do_process_madvise(pidfd, &iter, behavior, flags);
 		kfree(iov);
 	}
 	return ret;
-- 
2.26.2.761.g0e0b3e54be-goog