transfering pages from user space to user space

[Pablo Pessolani]

Hi Yann:

Reading the patch source code I find that the pages from one user space to other are "copied" 
102                 if (vm_write)
103                         ret = copy_from_user(target_kaddr,
104                                              lvec[*lvec_current].iov_base
105                                              + *lvec_offset,
106                                              bytes_to_copy);
107                 else
108                         ret = copy_to_user(lvec[*lvec_current].iov_base
109                                            + *lvec_offset,
110                                            target_kaddr, bytes_to_copy);
 
The code I wrote (very similar to this) also copies page contents. But my interest is transfering pages (zero-copy).
Regards.
PAP
 

> Subject: Re: transfering pages from user space to user space
> From: ydroneaud at opteya.com
> To: ppessolani at hotmail.com
> Date: Thu, 6 Dec 2012 10:30:27 +0100
> CC: kernelnewbies at kernelnewbies.org
> 
> Hi,
> 
> Le mercredi 05 décembre 2012 à 22:47 -0300, Pablo Pessolani a écrit :
> > Hi:
> > I am working on a project to copy (page aligned) the
> > buffer content of one process to the buffer of other process.
> > 
> > Now I resolved this issue using copy_page() but, analizing
> > performance with different buffer sizes, the "copy_page" becames the
> > critical time component and limiting factor.
> 
> This sounds a lot like "Cross Memory Support" (eg CROSS_MEMORY_ATTACH
> option) introduced in Linux 3.2:
> 
> http://kernelnewbies.org/Linux_3.2#head-a5e26c6275e85a5c9c41873fbab96bd38d934b72
> 
> Cross Memory Support add two syscalls:
> - process_vm_readv() : read from a process memory
> - process_vm_writev() : write to a process memory
> 
> Details can be found here:
> 
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=fcf634098c00dd9cd247447368495f0b79be12d1
> 
> And documentation here:
> 
> http://www.man7.org/linux/man-pages/man2/process_vm_readv.2.html
> http://ozlabs.org/~cyeoh/cma/process_vm_readv.txt
> 
> > [...] The kernel I use is 2.6.32.
> > 
> 
> BTW, why use a kernel released 3 years ago for such new development ?
> Kernel 2.6.32 was released the 3rd of december 2009. Even the -rt
> project switch to newer kernel (eg. no less than 3.0, and up to 3.4),
> see http://rt.wiki.kernel.org/
> 
> You should at least switch to a current long term support kernels, for
> example Linux 3.4. See
> http://www.kroah.com/log/linux/stable-status-08-2012.html
> 
> Regards
> 
> -- 
> Yann Droneaud
> OPTEYA
> 
> 
> 
> 
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