On Fri, 22 Mar 2024 at 22:08, David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 22.03.24 20:46, Miklos Szeredi wrote:
> > On Fri, 22 Mar 2024 at 16:41, David Hildenbrand <david@xxxxxxxxxx> wrote:
> >
> >> But at least the vmsplice() just seems to work. Which is weird, because
> >> GUP-fast should not apply (page not faulted in?)
> >
> > But it is faulted in, and that indeed seems to be the root cause.
>
> secretmem mmap() won't populate the page tables. So it's not faulted in yet.
>
> When we GUP via vmsplice, GUP-fast should not find it in the page tables
> and fallback to slow GUP.
>
> There, we seem to pass check_vma_flags(), trigger faultin_page() to
> fault it in, and then find it via follow_page_mask().
>
> ... and I wonder how we manage to skip check_vma_flags(), or otherwise
> managed to GUP it.
>
> vmsplice() should, in theory, never succeed here.
>
> Weird :/
>
> > Improved repro:
> >
> > #define _GNU_SOURCE
> >
> > #include <fcntl.h>
> > #include <unistd.h>
> > #include <stdio.h>
> > #include <errno.h>
> > #include <sys/mman.h>
> > #include <sys/syscall.h>
> >
> > int main(void)
> > {
> > int fd1, fd2;
> > int pip[2];
> > struct iovec iov;
> > char *addr;
> > int ret;
> >
> > fd1 = syscall(__NR_memfd_secret, 0);
> > addr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd1, 0);
> > ftruncate(fd1, 7);
> > addr[0] = 1; /* fault in page */
Here the page is faulted in and GUP-fast will find it. It's not in
the kernel page table, but it is in the user page table, which is what
matter for GUP.
Thanks,
Miklos
