Yes, I see from Lorenzo's reply that there is apparently some history to
this (maybe it's all nicely summarized in the cover letter / this patch,
have to dig further).
Not sure yet what the problem is, I would have thought it's all protected by
the PTL, and concurrent faults are user space doing something stupid and
we'd detect it.
The looping mechanism is fine for dealing with concurrent faults. There's
no actual _race_ due to PTL, it's just that a user could repeatedly
populate stuff stupidly in a range that is meant to have poison markers put
in.
It's not likely and would be kind of an abusive of the interface, and it'd
really be a process just hurting itself.
In nearly all cases you won't zap at all. The whole point is it's
optimistic. In 99.99% of others you zap once...
Have to do some more reading on this.
May I suggest a book on the history of the prodigy?
I'd normally agree with the KIS principle, but..
We can always implement support for that later if
it would either mean later we change behavior (installing guards on
non-zapped PTEs would have to be an error now but maybe start working later,
which is user observable change thus can break somebody)
really required (leave behavior open when documenting).
and leaving it open when documenting doesn't really mean anything for the
"we don't break userspace" promise vs what the implementation actually does.
Not quite I think. You could start return -EEXIST or -EOPNOTSUPP and
document that this can change in the future to succeed if there is
something. User space can sense support.
Yeah I mean originally I had a -EAGAIN which was sort of equivalent of this
but Jann pointed out you're just shifting work to userland who would loop
and repeat.
I just don't see why we'd do this.
In fact I was looking at the series and thinking 'wow it's actually a
really small delta' and being proud but... still not KIS enough apparently
;)
Something failing that at one point starts working is not really breaking
user space, unless someone really *wants* to fail if there is already
something (e.g., concurrent fault -> bail out instead of hiding it).
Of course, a more elegant solution would be GUARD_INSTALL vs.
GUARD_FORCE_INSTALL.
.. but again, there seems to be more history to this.
I don't think there's really any value in that. There's just no sensible
situation in which a user would care about this I don't think.
And if you're saying 'hey do MADV_DONTNEED if this fails and keep trying!'
then why not just do that in the kernel?
Trying to explain to a user 'hey this is for installing guard pages but if
there's a facing fault it'll fail and that could keep happening and then
you'll have to zap and maybe in a loop' just... seems like a bloody awful
interface?