Support for Intel X1000

[Kinsella, Ray]

> Is there any known way to change this behavior and eliminate the CoW?

Apologies - to answer my own question ...

"Memory locks are not inherited by a child created via fork(2) and are
automatically removed (unlocked) during an execve(2) or when the process
terminates. The mlockall() MCL_FUTURE setting is not inherited by a
child created via fork(2) and is cleared during an execve(2)."

mlockall(MCL_CURRENT) after the fork,  triggers the CoW upfront.

PARENT: locking
fork to PARENT
fork to CHILD0 <- mlockall(MCL_CURRENT)
PARENT: 1 : 385 : 0xb7786000 : 0xdce
CHILD0: 0 : 386 : 0xb7786000 : 0xa630
PARENT: release
CHILD0: locking
CHILD0: 1 : 386 : 0xb7786000 : 0xa630
PARENT: 0 : 385 : 0xb7786000 : 0xdce

Ray K

On Tue, 2015-05-26 at 15:16 +0000, Kinsella, Ray wrote:
> On Wed, 2015-05-20 at 10:15 -0400, dalias@libc.org wrote:
> > On Wed, May 20, 2015 at 01:54:13PM +0000, Kinsella, Ray wrote:
> >  
> > If this is true, it's a bug in the implementation of mlockall. The
> > whole point of memory locking is to prevent the need to allocate
> > memory at page fault time, which matters for multiple reasons,
> > including at least:
> 
> Hi Rich, 
> 
> ok ... I am seeing slightly different behavior.
> 
> From the mlockall man page ....
> "MCL_FUTURE  Lock all pages which will become mapped into the address
> space of the process in the future.  These could be for instance new
> pages  required  by a growing heap and stack as well as new
> memory-mapped files or shared memory regions".
> 
> So I wrote some test code ...
> 1. mlockall(MCL_FUTURE) upfront.
> 2. mmap a page, with MAP_PRIVATE | MAP_ANONYMOUS, PROT_READ |
> PROT_WRITE.
> 3. print out the virt to physical page mapping
> 4. fork() 
> 5. print out the virt to physical page mapping (parent and child).
> 
> What I see is that same physical page is mapped into both parent and
> child process space until a process writes to it - at which point a CoW
> Page Fault occurs. A new physical page is then mapped into the writing
> process address space. 
> 
> INIT: <- mlockall(MCL_FUTURE) && mmap 1 page
> PARENT: locking <- parent process writes to CoW Page. 
> fork to PARENT <- fork(), page is set as CoW
> fork to CHILD0
> PARENT: 1 : 13560 : 0xf7760000 : 0x399d5 <- PID, Page Virt Addr, Page
> Phy Addr
> CHILD0: 0 : 13561 : 0xf7760000 : 0x399d5
> PARENT: release <- 1st write to 0x399d5 after fork(), causes #PF
> allocating 0x6d548. 
> CHILD0: locking
> PARENT: 0 : 13560 : 0xf7760000 : 0x6d548 <- PID, Virt Addr, Phy Addr
> CHILD0: 1 : 13561 : 0xf7760000 : 0x399d5
> ...
> 
> Is there any known way to change this behavior and eliminate the CoW?
> 
> Ray K
> 
> 
> 
> 
> 
> 
>