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

[William Kucharski <kucharsk@gmail.com>]

I would think this should be the case; certainly it seems to be a more effective approach than having to manually enable sharing via the API in every case or via changes to ld.so.

If anything it might be useful to have an API for shutting it off, though there are already multiple areas where the system shares resources in ways that cannot be shut off by user action.

> On Jan 25, 2022, at 04:41, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> 
> 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
>