Re: [PATCH v2 5/5] ioctl_userfaultfd.2: document new UFFDIO_POISON ioctl

[Axel Rasmussen <axelrasmussen@google.com>]

On Sun, Oct 8, 2023 at 3:23 PM Alejandro Colomar <alx@kernel.org> wrote:
>
> Hi Axel,
>
> On Tue, Oct 03, 2023 at 12:45:47PM -0700, Axel Rasmussen wrote:
> > This is a new feature recently added to the kernel. So, document the new
> > ioctl the same way we do other UFFDIO_* ioctls.
> >
> > Also note the corresponding new ioctl flag we can return in reponse to a
> > UFFDIO_REGISTER call.
> >
> > Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
> > ---
> >  man2/ioctl_userfaultfd.2 | 112 +++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 112 insertions(+)
> >
> > diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
> > index 95d69f773..6b6980d4a 100644
> > --- a/man2/ioctl_userfaultfd.2
> > +++ b/man2/ioctl_userfaultfd.2
> > @@ -380,6 +380,11 @@ operation is supported.
> >  The
> >  .B UFFDIO_CONTINUE
> >  operation is supported.
> > +.TP
> > +.B 1 << _UFFDIO_POISON
> > +The
> > +.B UFFDIO_POISON
> > +operation is supported.
> >  .PP
> >  This
> >  .BR ioctl (2)
> > @@ -890,6 +895,113 @@ The faulting process has exited at the time of a
> >  .B UFFDIO_CONTINUE
> >  operation.
> >  .\"
> > +.SS UFFDIO_POISON
> > +(Since Linux 6.6.)
> > +Mark an address range as "poisoned".
> > +Future accesses to these addresses will raise a
> > +.B SIGBUS
> > +signal.
> > +Unlike
> > +.B MADV_HWPOISON
> > +this works by installing page table entries,
> > +rather than "really" poisoning the underlying physical pages.
> > +This means it only affects this particular address space.
> > +.PP
> > +The
> > +.I argp
> > +argument is a pointer to a
> > +.I uffdio_continue
> > +structure as shown below:
> > +.PP
> > +.in +4n
> > +.EX
> > +struct uffdio_poison {
> > +     struct uffdio_range range;
> > +                     /* Range to install poison PTE markers in */
> > +     __u64 mode;     /* Flags controlling the behavior of poison */
> > +     __s64 updated;  /* Number of bytes poisoned, or negated error */
> > +};
> > +.EE
> > +.in
> > +.PP
> > +The following value may be bitwise ORed in
> > +.I mode
> > +to change the behavior of the
> > +.B UFFDIO_POISON
> > +operation:
> > +.TP
> > +.B UFFDIO_POISON_MODE_DONTWAKE
> > +Do not wake up the thread that waits for page-fault resolution.
> > +.PP
> > +The
> > +.I updated
> > +field is used by the kernel
> > +to return the number of bytes that were actually poisoned,
> > +or an error in the same manner as
> > +.BR UFFDIO_COPY .
> > +If the value returned in the
> > +.I updated
> > +field doesn't match the value that was specified in
> > +.IR range.len ,
> > +the operation fails with the error
> > +.BR EAGAIN .
> > +The
> > +.I updated
> > +field is output-only;
> > +it is not read by the
> > +.B UFFDIO_POISON
> > +operation.
> > +.PP
> > +This
> > +.BR ioctl (2)
> > +operation returns 0 on success.
> > +In this case,
> > +the entire area was poisoned.
> > +On error, \-1 is returned and
> > +.I errno
> > +is set to indicate the error.
> > +Possible errors include:
> > +.TP
> > +.B EAGAIN
> > +The number of bytes mapped
> > +(i.e., the value returned in the
> > +.I updated
> > +field)
> > +does not equal the value that was specified in the
> > +.I range.len
> > +field.
> > +.TP
> > +.B EINVAL
> > +Either
> > +.I range.start
> > +or
> > +.I range.len
> > +was not a multiple of the system page size; or
> > +.I range.len
> > +was zero; or the range specified was invalid.
> > +.TP
> > +.B EINVAL
> > +An invalid bit was specified in the
> > +.I mode
> > +field.
> > +.TP
> > +.B EEXIST
>
> Any reasons for this order, or should we use alphabetic order?

This is the order the conditions are checked in code, but I agree
alphabetic order is better. :) I'll send a v3.

>
> Thanks,
> Alex
>
> > +One or more pages were already mapped in the given range.
> > +.TP
> > +.B ENOENT
> > +The faulting process has changed its virtual memory layout simultaneously with
> > +an outstanding
> > +.B UFFDIO_POISON
> > +operation.
> > +.TP
> > +.B ENOMEM
> > +Allocating memory for page table entries failed.
> > +.TP
> > +.B ESRCH
> > +The faulting process has exited at the time of a
> > +.B UFFDIO_POISON
> > +operation.
> > +.\"
> >  .SH RETURN VALUE
> >  See descriptions of the individual operations, above.
> >  .SH ERRORS
> > --
> > 2.42.0.609.gbb76f46606-goog
> >
>
> --
> <https://www.alejandro-colomar.es/>