Hi, Adam, On 15.05.2019 22:38, Adam Borowski wrote: > On Wed, May 15, 2019 at 06:11:15PM +0300, 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: > > I wonder, why not optimize the existing interfaces to do zero copy if > properly aligned? No need for a new syscall, and old code would immediately > benefit. Because, this is just not possible. You can't zero copy anonymous pages of a process to pages of a remote process, when they are different pages. >> 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. > > That'll handle all of your above problems, except for making pages > subject to CoW if written to. But if making pages writeably shared is > desired, the old functions have a "flags" argument that doesn't yet have a > single bit defined. Kirill
