[RFC PATCH 1/1] mm/mremap: add MREMAP_MIRROR flag for existing mirroring functionality

[Mike Kravetz <mike.kravetz@xxxxxxxxxx>]

The mremap system call has the ability to 'mirror' parts of an existing
mapping.  To do so, it creates a new mapping that maps the same pages as
the original mapping, just at a different virtual address.  This
functionality has existed since at least the 2.6 kernel.

This patch simply adds a new flag to mremap which will make this
functionality part of the API.  It maintains backward compatibility with
the existing way of requesting mirroring (old_size == 0).

If this new MREMAP_MIRROR flag is specified, then new_size must equal
old_size.  In addition, the MREMAP_MAYMOVE flag must be specified.

Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 include/uapi/linux/mman.h       |  5 +++--
 mm/mremap.c                     | 23 ++++++++++++++++-------
 tools/include/uapi/linux/mman.h |  5 +++--
 3 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
index ade4acd..6b3e0df 100644
--- a/include/uapi/linux/mman.h
+++ b/include/uapi/linux/mman.h
@@ -3,8 +3,9 @@
 
 #include <asm/mman.h>
 
-#define MREMAP_MAYMOVE	1
-#define MREMAP_FIXED	2
+#define MREMAP_MAYMOVE	0x01
+#define MREMAP_FIXED	0x02
+#define MREMAP_MIRROR	0x04
 
 #define OVERCOMMIT_GUESS		0
 #define OVERCOMMIT_ALWAYS		1
diff --git a/mm/mremap.c b/mm/mremap.c
index cd8a1b1..f18ab36 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -516,10 +516,11 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 	struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
 	LIST_HEAD(uf_unmap);
 
-	if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE))
+	if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_MIRROR))
 		return ret;
 
-	if (flags & MREMAP_FIXED && !(flags & MREMAP_MAYMOVE))
+	if ((flags & MREMAP_FIXED || flags & MREMAP_MIRROR) &&
+	    !(flags & MREMAP_MAYMOVE))
 		return ret;
 
 	if (offset_in_page(addr))
@@ -528,14 +529,22 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 	old_len = PAGE_ALIGN(old_len);
 	new_len = PAGE_ALIGN(new_len);
 
-	/*
-	 * We allow a zero old-len as a special case
-	 * for DOS-emu "duplicate shm area" thing. But
-	 * a zero new-len is nonsensical.
-	 */
+	/* A zero new-len is nonsensical. */
 	if (!new_len)
 		return ret;
 
+	/*
+	 * For backward compatibility, we allow a zero old-len to imply
+	 * mirroring.  This was originally a special case for DOS-emu.
+	 */
+	if (!old_len)
+		flags |= MREMAP_MIRROR;
+	else if (flags & MREMAP_MIRROR) {
+		if (old_len != new_len)
+			return ret;
+		old_len = 0;
+	}
+
 	if (down_write_killable(&current->mm->mmap_sem))
 		return -EINTR;
 
diff --git a/tools/include/uapi/linux/mman.h b/tools/include/uapi/linux/mman.h
index 81d8edf..069f7a5 100644
--- a/tools/include/uapi/linux/mman.h
+++ b/tools/include/uapi/linux/mman.h
@@ -3,8 +3,9 @@
 
 #include <uapi/asm/mman.h>
 
-#define MREMAP_MAYMOVE	1
-#define MREMAP_FIXED	2
+#define MREMAP_MAYMOVE	0x01
+#define MREMAP_FIXED	0x02
+#define MREMAP_MIRROR	0x04
 
 #define OVERCOMMIT_GUESS		0
 #define OVERCOMMIT_ALWAYS		1
-- 
2.7.5

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