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Date:   Thu, 16 May 2019 15:30:34 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     akpm@linux-foundation.org, dan.j.williams@intel.com,
        keith.busch@intel.com, kirill.shutemov@linux.intel.com,
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        linux-api@vger.kernel.org
Subject: Re: [PATCH RFC 0/5] mm: process_vm_mmap() -- syscall for duplication
 a process mapping
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[You are defining a new user visible API, please always add linux-api
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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