Re: [RFC PATCH 0/6] Add support for shared PTEs across processes

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Jan 18, 2022 at 02:19:12PM -0700, Khalid Aziz wrote:
> Example Code
> ============
> 
> Snippet of the code that a donor process would run looks like below:
> 
> -----------------
>         addr = mmap((void *)TB(2), GB(512), PROT_READ | PROT_WRITE,
>                         MAP_SHARED | MAP_ANONYMOUS, 0, 0);
>         if (addr == MAP_FAILED)
>                 perror("ERROR: mmap failed");
> 
>         err = syscall(MSHARE_SYSCALL, "testregion", (void *)TB(2),
> 			GB(512), O_CREAT|O_RDWR|O_EXCL, 600);
>         if (err < 0) {
>                 perror("mshare() syscall failed");
>                 exit(1);
>         }
> 
>         strncpy(addr, "Some random shared text",
> 			sizeof("Some random shared text"));
> -----------------
> 
> Snippet of code that a consumer process would execute looks like:
> 
> -----------------
>         fd = open("testregion", O_RDONLY);
>         if (fd < 0) {
>                 perror("open failed");
>                 exit(1);
>         }
> 
>         if ((count = read(fd, &mshare_info, sizeof(mshare_info)) > 0))
>                 printf("INFO: %ld bytes shared at addr %lx \n",
> 				mshare_info[1], mshare_info[0]);
>         else
>                 perror("read failed");
> 
>         close(fd);
> 
>         addr = (char *)mshare_info[0];
>         err = syscall(MSHARE_SYSCALL, "testregion", (void *)mshare_info[0],
> 			mshare_info[1], O_RDWR, 600);
>         if (err < 0) {
>                 perror("mshare() syscall failed");
>                 exit(1);
>         }
> 
>         printf("Guest mmap at %px:\n", addr);
>         printf("%s\n", addr);
> 	printf("\nDone\n");
> 
>         err = syscall(MSHARE_UNLINK_SYSCALL, "testregion");
>         if (err < 0) {
>                 perror("mshare_unlink() failed");
>                 exit(1);
>         }
> -----------------

I wounder if we can get away with zero-API here: we can transparently
create/use shared page tables for any inode on mmap(MAP_SHARED) as long as
size and alignment is sutiable. Page tables will be linked to the inode
and will be freed when the last of such mapping will go away. I don't see
a need in new syscalls of flags to existing one.

-- 
 Kirill A. Shutemov