Cross Memory Attach

The basic idea behind cross memory attach is to allow MPI programs doing
intra-node communication to do a single copy of the message rather than a
double copy of the message via shared memory.

The following patch attempts to achieve this by allowing a destination
process, given an address and size from a source process, to copy memory
directly from the source process into its own address space via a system
call.  There is also a symmetrical ability to copy from the current
process's address space into a destination process's address space.

- Use of /proc/pid/mem has been considered, but there are issues with
  using it:
  - Does not allow for specifying iovecs for both src and dest, assuming
    preadv or pwritev was implemented either the area read from or
  written to would need to be contiguous.
  - Currently mem_read allows only processes who are currently
  ptrace'ing the target and are still able to ptrace the target to read
  from the target. This check could possibly be moved to the open call,
  but its not clear exactly what race this restriction is stopping
  (reason  appears to have been lost)
  - Having to send the fd of /proc/self/mem via SCM_RIGHTS on unix
  domain socket is a bit ugly from a userspace point of view,
  especially when you may have hundreds if not (eventually) thousands
  of processes  that all need to do this with each other
  - Doesn't allow for some future use of the interface we would like to
  consider adding in the future (see below)
  - Interestingly reading from /proc/pid/mem currently actually
  involves two copies! (But this could be fixed pretty easily)

As mentioned previously use of vmsplice instead was considered, but has
problems.  Since you need the reader and writer working co-operatively if
the pipe is not drained then you block.  Which requires some wrapping to
do non blocking on the send side or polling on the receive.  In all to all
communication it requires ordering otherwise you can deadlock.  And in the
example of many MPI tasks writing to one MPI task vmsplice serialises the
copying.

There are some cases of MPI collectives where even a single copy interface
does not get us the performance gain we could.  For example in an
MPI_Reduce rather than copy the data from the source we would like to
instead use it directly in a mathops (say the reduce is doing a sum) as
this would save us doing a copy.  We don't need to keep a copy of the data
from the source.  I haven't implemented this, but I think this interface
could in the future do all this through the use of the flags - eg could
specify the math operation and type and the kernel rather than just
copying the data would apply the specified operation between the source
and destination and store it in the destination.

Although we don't have a "second user" of the interface (though I've had
some nibbles from people who may be interested in using it for intra
process messaging which is not MPI).  This interface is something which
hardware vendors are already doing for their custom drivers to implement
fast local communication.  And so in addition to this being useful for
OpenMPI it would mean the driver maintainers don't have to fix things up
when the mm changes.

There was some discussion about how much faster a true zero copy would
go. Here's a link back to the email with some testing I did on that:

http://marc.info/?l=linux-mm&m=130105930902915&w=2

There is a basic man page for the proposed interface here:

http://ozlabs.org/~cyeoh/cma/process_vm_readv.txt

This has been implemented for x86 and powerpc, other architecture should
mainly (I think) just need to add syscall numbers for the process_vm_readv
and process_vm_writev. There are 32 bit compatibility versions for
64-bit kernels.

For arch maintainers there are some simple tests to be able to quickly
verify that the syscalls are working correctly here:

http://ozlabs.org/~cyeoh/cma/cma-test-20110718.tgz

Signed-off-by: Chris Yeoh <yeohc@au1.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: <linux-man@vger.kernel.org>
Cc: <linux-arch@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
aio: allocate kiocbs in batches

In testing aio on a fast storage device, I found that the context lock
takes up a fair amount of cpu time in the I/O submission path.  The reason
is that we take it for every I/O submitted (see __aio_get_req).  Since we
know how many I/Os are passed to io_submit, we can preallocate the kiocbs
in batches, reducing the number of times we take and release the lock.

In my testing, I was able to reduce the amount of time spent in
_raw_spin_lock_irq by .56% (average of 3 runs).  The command I used to
test this was:

   aio-stress -O -o 2 -o 3 -r 8 -d 128 -b 32 -i 32 -s 16384 <dev>

I also tested the patch with various numbers of events passed to
io_submit, and I ran the xfstests aio group of tests to ensure I didn't
break anything.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Cc: Daniel Ehrenberg <dehrenberg@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Unused iocbs in a batch should not be accounted as active.

Since commit 080d676de095 ("aio: allocate kiocbs in batches") iocbs are
allocated in a batch during processing of first iocbs.  All iocbs in a
batch are automatically added to ctx->active_reqs list and accounted in
ctx->reqs_active.

If one (not the last one) of iocbs submitted by an user fails, further
iocbs are not processed, but they are still present in ctx->active_reqs
and accounted in ctx->reqs_active.  This causes process to stuck in a D
state in wait_for_all_aios() on exit since ctx->reqs_active will never
go down to zero.  Furthermore since kiocb_batch_free() frees iocb
without removing it from active_reqs list the list become corrupted
which may cause oops.

Fix this by removing iocb from ctx->active_reqs and updating
ctx->reqs_active in kiocb_batch_free().

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Cc: stable@kernel.org   # 3.2
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
aio: wake up waiters when freeing unused kiocbs

Bart Van Assche reported a hung fio process when either hot-removing
storage or when interrupting the fio process itself.  The (pruned) call
trace for the latter looks like so:

  fio             D 0000000000000001     0  6849   6848 0x00000004
   ffff880092541b88 0000000000000046 ffff880000000000 ffff88012fa11dc0
   ffff88012404be70 ffff880092541fd8 ffff880092541fd8 ffff880092541fd8
   ffff880128b894d0 ffff88012404be70 ffff880092541b88 000000018106f24d
  Call Trace:
    schedule+0x3f/0x60
    io_schedule+0x8f/0xd0
    wait_for_all_aios+0xc0/0x100
    exit_aio+0x55/0xc0
    mmput+0x2d/0x110
    exit_mm+0x10d/0x130
    do_exit+0x671/0x860
    do_group_exit+0x44/0xb0
    get_signal_to_deliver+0x218/0x5a0
    do_signal+0x65/0x700
    do_notify_resume+0x65/0x80
    int_signal+0x12/0x17

The problem lies with the allocation batching code.  It will
opportunistically allocate kiocbs, and then trim back the list of iocbs
when there is not enough room in the completion ring to hold all of the
events.

In the case above, what happens is that the pruning back of events ends
up freeing up the last active request and the context is marked as dead,
so it is thus responsible for waking up waiters.  Unfortunately, the
code does not check for this condition, so we end up with a hung task.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Reported-by: Bart Van Assche <bvanassche@acm.org>
Tested-by: Bart Van Assche <bvanassche@acm.org>
Cc: <stable@kernel.org>		[3.2.x only]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
aio: don't bother with async freeing on failure in ioctx_alloc()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
aio: aio_nr decrements don't need to be delayed

we can do that right in __put_ioctx(); as the result, the loop
in ioctx_alloc() can be killed.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
aio: aio_nr_lock is taken only synchronously now

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
aio: use cancel_delayed_work_sync()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
aio: don't bother with cancel_delayed_work() in exit_aio()

__put_ioctx() will cover it anyway.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
aio: fix the comment in aio_kick_handler()

	It should've been changed when queue_work() became
queue_delayed_work(..., 0) in there.  It's always had been
about not needing a delay, not about not using specific
function...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
aio: merge aio_cancel_all() with wait_for_all_aios()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
aio: take final put_ioctx() into callers of io_destroy()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
VM: add "vm_munmap()" helper function

Like the vm_brk() function, this is the same as "do_munmap()", except it
does the VM locking for the caller.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
aio: don't bother with unmapping when aio_free_ring() is coming from exit_aio()

... since exit_mmap() is coming and it will munmap() everything anyway.
In all other cases aio_free_ring() has ctx->mm == current->mm; moreover,
all other callers of vm_munmap() have mm == current->mm, so this will
allow us to get rid of mm argument of vm_munmap().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
kill mm argument of vm_munmap()

it's always current->mm

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
aio/vfs: cleanup of rw_copy_check_uvector() and compat_rw_copy_check_uvector()

A cleanup of rw_copy_check_uvector and compat_rw_copy_check_uvector after
changes made to support CMA in an earlier patch.

Rather than having an additional check_access parameter to these
functions, the first paramater type is overloaded to allow the caller to
specify CHECK_IOVEC_ONLY which means check that the contents of the iovec
are valid, but do not check the memory that they point to.  This is used
by process_vm_readv/writev where we need to validate that a iovec passed
to the syscall is valid but do not want to check the memory that it points
to at this point because it refers to an address space in another process.

Signed-off-by: Chris Yeoh <yeohc@au1.ibm.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
aio: now fput() is OK from interrupt context; get rid of manual delayed __fput()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Kernel 7.29 ICS-JB-MALI + config change to fix ICS ROMS.