Davidlohr Bueso | 18 Apr 03:25 2014

[PATCH v3] ipc,shm: disable shmmax and shmall by default

The default size for shmmax is, and always has been, 32Mb.
Today, this value is rather small, making users have to
increase it via sysctl, which can cause unnecessary work and
userspace application workarounds. Ie:


Unix has historically required setting these limits for shared
memory, and Linux inherited such behavior. The consequence of this
is added complexity for users and administrators. One very common
example are Database setup/installation documents and scripts,
where users must manually calculate the values for these limits.
This also requires (some) knowledge of how the underlying memory
management works, thus causing, in many occasions, the limits to
just be flat out wrong. Disabling these limits sooner could have
saved companies a lot of time, headaches and money for support.
But it's never too late, simplify users life now.

Instead of choosing yet another arbitrary value, larger than 32Mb,
this patch disables the use of both shmmax and shmall by default,
allowing users to create segments of unlimited sizes. Users and
applications that already explicitly set these values through sysctl
are left untouched, and thus does not change any of the behavior.

So a value of 0 bytes or pages, for shmmax and shmall, respectively,
implies unlimited memory, as opposed to disabling sysv shared memory.
This is safe as 0 cannot possibly be used previously as SHMMIN is
hardcoded to 1 and cannot be modified. This change will of course
be reflected in shmctl(SHM_STAT) calls. Any application that does
preliminary checking of the size of shmmax, must also check for
(Continue reading)

Ming Zhao | 13 Apr 07:59 2014

EPOLLOUT is triggered after sending data on UDP socket without EAGAIN

Hi there,

I'm not sure whether this is an epoll bug:

- I have a UDP socket which is non-blocking and watched by epoll with
- EPOLLOUT is triggered after registering the file descriptor with
epoll_add, which is expected.
- After calling sendto on the UDP socket with a very small message(5
bytes), sendto returns the exact number of bytes sent and no EAGAIN is
- *but* calling epoll_wait still returns EPOLLOUT.

Shouldn't that EPOLLOUT be only triggered when an EAGAIN is generated
after any writing operation?

Here is the code I use:
We have very simple wrapper for socket and epoll operations, and you
can easily guess what they're doing

TEST(EpollService, UdpWriteable) {

  Socket client(Socket::UDP());

  Socket server(Socket::UDP());

  // Ask os to pick random port
  CHECK_ERR(server.Bind(SocketAddress(SocketAddress::LOOPBACK, 0)));

(Continue reading)

Steven Stewart-Gallus | 1 Apr 06:41 2014

How about allowing processes to expose memory for cross memory attaching?


A major limitation of process_vm_writev and process_vm_readv is that
they have the same permission constraints as ptrace. How about allowing
processes to expose memory for reading with process_vm_readv or
writing with process_vm_writev?

I think a good way of doing this might be to add flags to
process_vm_writev and process_vm_readv to make them temporarily
publish a vector of memory to another process until such memory is
read or written from with process_vm_writev or process_vm_readv once.
For example, one would invoke the process_vm functionality like so to
publish for reading the vector of memory in local_iov to the process
identified by pid:

if (-1 == process_vm_writev(pid,
                           local_iov, ARRAY_SIZE(local_iov),
                           NULL, 0,
                           PROC_VM_PUBLISH)) { // ...

The interface would be simpler if the functions blocked but then for
every EINTR handling loop there would need to be an EPERM loop on the
other side.

Finally, I'm not sure of what the semantics of that interface would be
in the presence of multithreading or shared memory. And, I'm not sure
how usable this interface would be for fast message passing which is
the whole intent of the process_vm functions (I'm using these
functions directly for my application and this interface seems like it
work fine with my application but I'm not sure it would work for
(Continue reading)

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Alex Thorlton | 25 Feb 21:54 2014

[PATCHv4 0/3] [RESEND] mm, thp: Add mm flag to control THP

(First send had too big of a cc list to make it into all the mailing lists.)

This patch is based on some of my work combined with some
suggestions/patches given by Oleg Nesterov.  The main goal here is to
add a prctl switch to allow us to disable to THP on a per mm_struct

Changes for v4:

* Added a bit of documentation for flag changes
* Changed the prctl switch code so that get/set operations are
  processed in separate cases
* Added information about the new flags to the user-facing prctl docs
  (this is in a separate patch, everyone will be cc'd there as well)

The main motivation behind this patch is to provide a way to disable THP
for jobs where the code cannot be modified, and using a malloc hook with
madvise is not an option (i.e. statically allocated data).  This patch
allows us to do just that, without affecting other jobs running on the

We need to do this sort of thing for jobs where THP hurts performance,
due to the possibility of increased remote memory accesses that can be
created by situations such as the following:

When you touch 1 byte of an untouched, contiguous 2MB chunk, a THP will
be handed out, and the THP will be stuck on whatever node the chunk was
originally referenced from.  If many remote nodes need to do work on
that same chunk, they'll be making remote accesses.

(Continue reading)

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(Continue reading)

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