At the end I tried the pidfd_getfd syscall that is working really fine and
giving me back a "clone" fd of an fd in that was opened from another
process. I tested it opening a text file in the first process  and after
cloning the fd , I could really read the file also in the second process.
Now the weird thing:
1) In the first process I allocate- a huge page, then get the fd
2) In the second process I get my "clone" fd and do an mmap, it works but
if I write on that memory, the first process cannot see what I wrote

int second_process(int remote_pid, int remote_mem_fd) {

        printf("remote_pid %d remote_mem_fd %d\n", remote_pid,
remote_mem_fd);
        int pidfd = syscall(__NR_pidfd_open, remote_pid, 0);

        int my_mem_fd = syscall(438, pidfd, remote_mem_fd, 0);
        printf("my_mem_fd %d\n", my_mem_fd);   // This is nice

        int flags = MAP_SHARED | MAP_HUGETLB | (30 << MAP_HUGE_SHIFT);
        uint64_t* addr = (uint64_t*) mmap(NULL, 1024 * 1024 * 1024,
PROT_READ|PROT_WRITE, flags, my_mem_fd, 0);
        if (addr == -1)
            perror("mmap");
        *addr = 0x0101010102020202;
}


Il giorno gio 14 apr 2022 alle ore 21:51 Antonio Di Bacco <
a.dibacco.ks@gmail.com> ha scritto:

>
>
> Il giorno gio 14 apr 2022 alle ore 21:01 Dmitry Kozlyuk <
> dmitry.kozliuk@gmail.com> ha scritto:
>
>> 2022-04-14 10:20 (UTC+0200), Antonio Di Bacco:
>> [...]
>> > Ok, after having a look to memif I managed to exchange the fd  between
>> the
>> > two processes and it works.
>> > Anyway the procedure seems a little bit clunky and I think I'm going to
>> use
>> > the new SYSCALL pidfd_getfd
>> > to achieve the same result.  In your opinion this method (getfd_pidfd)
>> > could also work if the two DPDK processes
>> > are inside different docker containers?
>>
>> Honestly, I've just learned about pidfd_getfd() from you.
>> But I know that containers use PID namespaces, so there's a question
>> how you will obtain the pidfd of a process in another container.
>>
>> In general, any method of sharing FD will work.
>> Remember that you also need offset and size.
>> Given that some channel is required to share those,
>> I think Unix domain socket is still the preferred way.
>>
>> > Or is there another mechanims like using handles to hugepages present in
>> > the filesystem to share between two
>> > different containers?
>>
>> FD is needed for mmap().
>> You need to either pass the FD or open() the same hugepage file by path.
>> I advise against using paths because they are not a part of DPDK API
>> contract.
>>
>
> Thank you very much Dmitry, your answers are always enlightening.
> I'm going to ask a different question on the dpdk.org about the best
> practice to share memory between two dpdk processes running in different
> containers.
>