On 07/27/2015 04:54 PM, Eric B Munson wrote:

On Mon, 27 Jul 2015, Vlastimil Babka wrote:

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We do actually have an MCL_LOCKED, we just call it MCL_CURRENT.  Would
you prefer that I match the name in mlock2() (add MLOCK_CURRENT
instead)?

Hm it's similar but not exactly the same, because MCL_FUTURE is not
the same as MLOCK_ONFAULT :) So MLOCK_CURRENT would be even more
confusing. Especially if mlockall(MCL_CURRENT | MCL_FUTURE) is OK,
but mlock2(MLOCK_LOCKED | MLOCK_ONFAULT) is invalid.

MLOCK_ONFAULT isn't meant to be the same as MCL_FUTURE, rather it is
meant to be the same as MCL_ONFAULT.  MCL_FUTURE only controls if the
locking policy will be applied to any new mappings made by this process,
not the locking policy itself.  The better comparison is MCL_CURRENT to
MLOCK_LOCK and MCL_ONFAULT to MLOCK_ONFAULT.  MCL_CURRENT and
MLOCK_LOCK do the same thing, only one requires a specific range of
addresses while the other works process wide.  This is why I suggested
changing MLOCK_LOCK to MLOCK_CURRENT.  It is an error to call
mlock2(MLOCK_LOCK | MLOCK_ONFAULT) just like it is an error to call
mlockall(MCL_CURRENT | MCL_ONFAULT).  The combinations do no make sense.

How is it an error to call mlockall(MCL_CURRENT | MCL_ONFAULT)? How else 
would you apply mlock2(MCL_ONFAULT) to all current mappings? Later below 
you use the same example and I don't think it's different by removing 
MLOCK_LOCKED flag.

This was all decided when VM_LOCKONFAULT was a separate state from
VM_LOCKED.  Now that VM_LOCKONFAULT is a modifier to VM_LOCKED and
cannot be specified independentally, it might make more sense to mirror
that relationship to userspace.  Which would lead to soemthing like the
following:

To lock and populate a region:
mlock2(start, len, 0);

To lock on fault a region:
mlock2(start, len, MLOCK_ONFAULT);

If LOCKONFAULT is seen as a modifier to mlock, then having the flags
argument as 0 mean do mlock classic makes more sense to me.

Yup that's what I was trying to suggest.

To mlock current on fault only:
mlockall(MCL_CURRENT | MCL_ONFAULT);

To mlock future on fault only:
mlockall(MCL_FUTURE | MCL_ONFAULT);

To lock everything on fault:
mlockall(MCL_CURRENT | MCL_FUTURE | MCL_ONFAULT);

I think I have talked myself into rewriting the set again :/

Sorry :) You could also wait a bit for more input than just from me...

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Finally, on the question of MAP_LOCKONFAULT, do you just dislike
MAP_LOCKED and do not want to see it extended, or is this a NAK on the
set if that patch is included.  I ask because I have to spin a V6 to get
the MLOCK flag declarations right, but I would prefer not to do a V7+.
If this is a NAK with, I can drop that patch and rework the tests to
cover without the mmap flag.  Otherwise I want to keep it, I have an
internal user that would like to see it added.

I don't want to NAK that patch if you think it's useful.