On Mon, Dec 02, 2019 at 10:21:03AM -0800, Sean Christopherson wrote:
> On Mon, Dec 02, 2019 at 09:48:43AM -0600, Haitao Huang wrote:
> > On Fri, 29 Nov 2019 17:13:14 -0600, Jarkko Sakkinen
> > <jarkko.sakkinen@xxxxxxxxxxxxxxx> wrote:
> >
> >
> > >+
> > >+ for (c = 0 ; c < addp.length; c += PAGE_SIZE) {
> > >+ if (signal_pending(current)) {
> > >+ ret = -ERESTARTSYS;
> > >+ break;
> > >+ }
> >
> > This IOC is not idempotent as pages EADDed at this point can not be
> > re-EADDed again. So we can't return ERESTARTSYS
>
> Ah, and now I remember why I opted for modifying the parameters directly
> instead of including a "number processed" field. Andy pointed out the
> ERESTARTSYS thing in the original multi-page add RFC[*], so presumably
> updating the params and returning ERESTARTSYS is legal/acceptable.
>
> [*] https://lkml.kernel.org/r/CALCETrUb4X9_L9RXKhmyNpfSCsbNodP=BfbfO8Fz_efq24jp8w@xxxxxxxxxxxxxx
There are exactly two primary options to pick from given the
non-idempotent nature:
A. Return zero (since we support partial operation with the changes I
did in v24).
B. Return -EINTR.
If we wanted to follow common syscall semantics in IO operations, then
the semantics would be a mix of these:
1. Return -EINTR if signals are pending before any pages got added.
2. Return 0 if at least one page got added.
This is how write() works for example according to the documentation
[1]. As far as the user code goes [2] I think for that it is best
idea to rely on semantics that developers are used to instead of
being clever here.
[1] http://man7.org/linux/man-pages/man2/write.2.html
[2] https://lkml.kernel.org/r/CALCETrUb4X9_L9RXKhmyNpfSCsbNodP=BfbfO8Fz_efq24jp8w@xxxxxxxxxxxxxx
/Jarkko
