linux-kernel - [PATCH 11/16] tmpfs: fcntl(fd, F_MEM

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Date:   Fri, 30 Jul 2021 00:57:56 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Hugh Dickins <hughd@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Michal Hocko <mhocko@...e.com>,
        Rik van Riel <riel@...riel.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexey Gladkov <legion@...nel.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Matthew Auld <matthew.auld@...el.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH 11/16] tmpfs: fcntl(fd, F_MEM_LOCKED) to test if memlocked

Though we have not yet found a compelling need to make the locked status
of a tmpfs file visible, and offer no tool to show it, the kernel ought
to be able to support such a tool: add the F_MEM_LOCKED fcntl, returning
-1 on failure (not tmpfs), 0 when not F_MEM_LOCKED, 1 when F_MEM_LOCKED.

Signed-off-by: Hugh Dickins <hughd@...gle.com>
---
 fs/fcntl.c                 | 1 +
 include/uapi/linux/fcntl.h | 1 +
 mm/shmem.c                 | 4 ++++
 3 files changed, 6 insertions(+)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index a3534764b50e..0d8dc723732d 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -439,6 +439,7 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 	case F_NOHUGEPAGE:
 	case F_MEM_LOCK:
 	case F_MEM_UNLOCK:
+	case F_MEM_LOCKED:
 		err = shmem_fcntl(filp, cmd, arg);
 		break;
 	default:
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 21dc969df0fd..012585e8c9ab 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -88,6 +88,7 @@
  */
 #define F_MEM_LOCK		(F_LINUX_SPECIFIC_BASE + 17)
 #define F_MEM_UNLOCK		(F_LINUX_SPECIFIC_BASE + 18)
+#define F_MEM_LOCKED		(F_LINUX_SPECIFIC_BASE + 19)
 
 /*
  * Types of directory notifications that may be requested.
diff --git a/mm/shmem.c b/mm/shmem.c
index ba9b9900287b..6e53dabe658b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2299,6 +2299,9 @@ static int shmem_memlock_fcntl(struct file *file, unsigned int cmd)
 	bool cleanup_mapping = false;
 	int retval = 0;
 
+	if (cmd == F_MEM_LOCKED)
+		return !!info->mlock_ucounts;
+
 	inode_lock(inode);
 	if (cmd == F_MEM_LOCK) {
 		if (!info->mlock_ucounts) {
@@ -2762,6 +2765,7 @@ long shmem_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
 		break;
 	case F_MEM_LOCK:
 	case F_MEM_UNLOCK:
+	case F_MEM_LOCKED:
 		error = shmem_memlock_fcntl(file, cmd);
 		break;
 	}
-- 
2.26.2