[PATCH] futex: Remove unnecessary warning from get_futex_key

[Mel Gorman <mgorman@suse.de>]

Commit 65d8fc777f6d ("futex: Remove requirement for lock_page() in
get_futex_key()") removed an unnecessary lock_page() with the side-effect
that page->mapping needed to be treated very carefully. Two defensive
warnings were added in case any assumption was missed and the first warning
assumed a correct application would not alter a mapping backing a futex key.
Since merging, it has not triggered for any unexpected case but Mark
Rutland reported the following bug triggering due to the first warning.

------------[ cut here ]------------
kernel BUG at kernel/futex.c:679!
Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
Modules linked in:
CPU: 0 PID: 3695 Comm: syz-executor1 Not tainted 4.13.0-rc3-00020-g307fec773ba3 #3
Hardware name: linux,dummy-virt (DT)
task: ffff80001e271780 task.stack: ffff000010908000
PC is at get_futex_key+0x6a4/0xcf0 kernel/futex.c:679
LR is at get_futex_key+0x6a4/0xcf0 kernel/futex.c:679
pc : [<ffff00000821ac14>] lr : [<ffff00000821ac14>] pstate: 80000145

The fact that it's a bug instead of a warning was due to an unrelated
issue but the warning itself triggered because the underlying mapping
changed. This is an application issue but from a kernel perspective it's
a recoverable situation and the warning is unnecessary so this patch
removes the warning. The warning may potentially be triggered with the
following test program from Mark although it may be necessary to adjust
NR_FUTEX_THREADS to be a value smaller than the number of CPUs in the system.

#include <linux/futex.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <sys/time.h>
#include <unistd.h>

#define NR_FUTEX_THREADS 16
pthread_t threads[NR_FUTEX_THREADS];

void *mem;

#define MEM_PROT        (PROT_READ | PROT_WRITE)
#define MEM_SIZE        65536

static int futex_wrapper(int *uaddr, int op, int val,
                         const struct timespec *timeout,
                         int *uaddr2, int val3)
{
        syscall(SYS_futex, uaddr, op, val, timeout, uaddr2, val3);
}

void *poll_futex(void *unused)
{
        for (;;) {
                futex_wrapper(mem, FUTEX_CMP_REQUEUE_PI, 1, NULL, mem + 4, 1);
        }
}

int main(int argc, char *argv[])
{
        int i;

        mem = mmap(NULL, MEM_SIZE, MEM_PROT,
                   MAP_SHARED | MAP_ANONYMOUS, -1, 0);

        printf("Mapping @ %p\n", mem);

        printf("Creating futex threads...\n");

        for (i = 0; i < NR_FUTEX_THREADS; i++)
                pthread_create(&threads[i], NULL, poll_futex, NULL);

        printf("Flipping mapping...\n");
        for (;;) {
                mmap(mem, MEM_SIZE, MEM_PROT,
                     MAP_FIXED | MAP_SHARED | MAP_ANONYMOUS, -1, 0);
        }

        return 0;
}

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: stable@vger.kernel.org # 4.7+
---
 kernel/futex.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 16dbe4c93895..f50b434756c1 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -670,13 +670,14 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
 		 * this reference was taken by ihold under the page lock
 		 * pinning the inode in place so i_lock was unnecessary. The
 		 * only way for this check to fail is if the inode was
-		 * truncated in parallel so warn for now if this happens.
+		 * truncated in parallel which is almost certainly an
+		 * application bug. In such a case, just retry.
 		 *
 		 * We are not calling into get_futex_key_refs() in file-backed
 		 * cases, therefore a successful atomic_inc return below will
 		 * guarantee that get_futex_key() will still imply smp_mb(); (B).
 		 */
-		if (WARN_ON_ONCE(!atomic_inc_not_zero(&inode->i_count))) {
+		if (!atomic_inc_not_zero(&inode->i_count)) {
 			rcu_read_unlock();
 			put_page(page);