another uninterruptable hang in sendfile

[Dmitry Vyukov]

Hello,

On commit 8005c49d9aea74d382f474ce11afbbc7d7130bec (Nov 15).

The program is:

// autogenerated by syzkaller (http://github.com/google/syzkaller)
#define _GNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h>
#include <sys/eventfd.h>
#include <sys/sendfile.h>
#include <fcntl.h>

#define SYS_memfd_create 319

int main()
{
        int r1 = eventfd(0, 0);
        int r2 = syscall(SYS_memfd_create, "", 0);
        unsigned long n = 1<<30;
        fallocate(r2, 0, 0, n);
        sendfile(r1, r2, 0, n);
        return 0;
}

It can be executed under non-privileged user and creates an unkillable
process. It probably will terminate eventually, but it can take
days/weeks. Note that "mm: make sendfile(2) killable"
296291cdd1629c308114504b850dc343eabc2782 is merged, so it is something
different. Though, probably similar.

Thank you.
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