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SubjectRe: [PATCH RFC 0/5] mm: process_vm_mmap() -- syscall for duplication a process mapping
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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@virtuozzo.com>

--
Michal Hocko
SUSE Labs

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