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.krav...@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(¤t->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