[You are defining a new user visible API, please always add linux-api mailing list - now done] On Wed 15-05-19 18:11:15, Kirill Tkhai wrote: > 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> -- Michal Hocko SUSE Labs
