This patchset adds a new syscall, which makes possible to clone a mapping from a process to another process. The syscall supplements the functionality provided by process_vm_writev() and process_vm_readv() syscalls, and it may be useful in many situation. For example, it allows to make a zero copy of data, when process_vm_writev() was previously used: struct iovec local_iov, remote_iov; void *buf; buf = mmap(NULL, n * PAGE_SIZE, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, ...); recv(sock, buf, n * PAGE_SIZE, 0); local_iov->iov_base = buf; local_iov->iov_len = n * PAGE_SIZE; remove_iov = ...; process_vm_writev(pid, &local_iov, 1, &remote_iov, 1 0); munmap(buf, n * PAGE_SIZE); (Note, that above completely ignores error handling) There are several problems with process_vm_writev() in this example: 1)it causes pagefault on remote process memory, and it forces allocation of a new page (if was not preallocated); 2)amount of memory for this example is doubled in a moment -- n pages in current and n pages in remote tasks are occupied at the same time; 3)received data has no a chance to be properly swapped for a long time. The third is the most critical in case of remote process touches the data pages some time after process_vm_writev() was made. Imagine, node is under memory pressure: a)kernel moves @buf pages into swap right after recv(); b)process_vm_writev() reads the data back from swap to pages; c)process_vm_writev() allocates duplicate pages in remote process and populates them; d)munmap() unmaps @buf; e)5 minutes later remote task touches data. In stages "a" and "b" kernel submits unneeded IO and makes system IO throughput worse. To make "b" and "c", kernel reclaims memory, and moves pages of some other processes to swap, so they have to read pages from swap back. Also, unneeded copying of pages is occured, while zero-copy is more preferred. We observe similar problem during online migration of big enough containers, when after doubling of container's size, the time increases 100 times. The system resides under high IO and throwing out of useful cashes. The proposed syscall aims to introduce an interface, which supplements currently existing process_vm_writev() and process_vm_readv(), and allows to solve the problem with anonymous memory transfer. The above example may be rewritten as: void *buf; buf = mmap(NULL, n * PAGE_SIZE, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, ...); recv(sock, buf, n * PAGE_SIZE, 0); /* Sign of @pid is direction: "from @pid task to current" or vice versa. */ process_vm_mmap(-pid, buf, n * PAGE_SIZE, remote_addr, PVMMAP_FIXED); munmap(buf, n * PAGE_SIZE); It is swap-friendly: in case of memory is swapped right after recv(), the syscall just copies pagetable entries like we do on fork(), so real access to pages does not occurs, and no IO is needed. No excess pages are reclaimed, and number of pages is not doubled. Also, zero-copy takes a place, and this also reduces overhead. The patchset does not introduce much new code, since we simply reuse existing copy_page_range() and copy_vma() functions. We extend copy_vma() to be able merge VMAs in remote task [2/5], and teach copy_page_range() to work with different local and remote addresses [3/5]. Patch [5/5] introduces the syscall logic, which mostly consists of sanity checks. The rest of patches are preparations. This syscall may be used for page servers like in example above, for migration (I assume, even virtual machines may want something like this), for zero-copy desiring users of process_vm_writev() and process_vm_readv(), for debug purposes, etc. It requires the same permittions like existing proc_vm_xxx() syscalls have. The tests I used may be obtained here: [1]https://gist.github.com/tkhai/198d32fdc001ec7812a5e1ccf091f275 [2]https://gist.github.com/tkhai/f52dbaeedad5a699f3fb386fda676562 --- Kirill Tkhai (5): mm: Add process_vm_mmap() syscall declaration mm: Extend copy_vma() mm: Extend copy_page_range() mm: Export round_hint_to_min() mm: Add process_vm_mmap() arch/x86/entry/syscalls/syscall_32.tbl | 1 arch/x86/entry/syscalls/syscall_64.tbl | 2 include/linux/huge_mm.h | 6 + include/linux/mm.h | 11 ++ include/linux/mm_types.h | 2 include/linux/mman.h | 14 +++ include/linux/syscalls.h | 5 + include/uapi/asm-generic/mman-common.h | 5 + include/uapi/asm-generic/unistd.h | 5 + init/Kconfig | 9 +- kernel/fork.c | 5 + kernel/sys_ni.c | 2 mm/huge_memory.c | 30 ++++-- mm/memory.c | 165 +++++++++++++++++++++----------- mm/mmap.c | 154 ++++++++++++++++++++++++++---- mm/mremap.c | 4 - mm/process_vm_access.c | 71 ++++++++++++++ 17 files changed, 392 insertions(+), 99 deletions(-) -- Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
