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Subject[RFC PATCH 1/1] mm/mremap: add MREMAP_MIRROR flag for existing mirroring functionality
Date
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@oracle.com>
---
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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