Re: [RESEND PATCH V2 0/3] Allow user to request memory to be locked on page fault

[Eric B Munson <emunson@akamai.com>]

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On Fri, 12 Jun 2015, Vlastimil Babka wrote:

> On 06/11/2015 09:34 PM, Andrew Morton wrote:
> >On Thu, 11 Jun 2015 15:21:30 -0400 Eric B Munson <emunson@akamai.com> wrote:
> >
> >>>Ditto mlockall(MCL_ONFAULT) followed by munlock().  I'm not sure
> >>>that even makes sense but the behaviour should be understood and
> >>>tested.
> >>
> >>I have extended the kselftest for lock-on-fault to try both of these
> >>scenarios and they work as expected.  The VMA is split and the VM
> >>flags are set appropriately for the resulting VMAs.
> >
> >munlock() should do vma merging as well.  I *think* we implemented
> >that.  More tests for you to add ;)
> >
> >How are you testing the vma merging and splitting, btw?  Parsing
> >the profcs files?
> >
> >>>What's missing here is a syscall to set VM_LOCKONFAULT on an
> >>>arbitrary range of memory - mlock() for lock-on-fault.  It's a
> >>>shame that mlock() didn't take a `mode' argument.  Perhaps we
> >>>should add such a syscall - that would make the mmap flag unneeded
> >>>but I suppose it should be kept for symmetry.
> >>
> >>Do you want such a system call as part of this set?  I would need some
> >>time to make sure I had thought through all the possible corners one
> >>could get into with such a call, so it would delay a V3 quite a bit.
> >>Otherwise I can send a V3 out immediately.
> >
> >I think the way to look at this is to pretend that mm/mlock.c doesn't
> >exist and ask "how should we design these features".
> >
> >And that would be:
> >
> >- mmap() takes a `flags' argument: MAP_LOCKED|MAP_LOCKONFAULT.
> 
> Note that the semantic of MAP_LOCKED can be subtly surprising:
> 
> "mlock(2) fails if the memory range cannot get populated to guarantee
> that no future major faults will happen on the range.
> mmap(MAP_LOCKED) on the other hand silently succeeds even if the
> range was populated only
> partially."
> 
> ( from http://marc.info/?l=linux-mm&m=143152790412727&w=2 )
> 
> So MAP_LOCKED can silently behave like MAP_LOCKONFAULT. While
> MAP_LOCKONFAULT doesn't suffer from such problem, I wonder if that's
> sufficient reason not to extend mmap by new mlock() flags that can
> be instead applied to the VMA after mmapping, using the proposed
> mlock2() with flags. So I think instead we could deprecate
> MAP_LOCKED more prominently. I doubt the overhead of calling the
> extra syscall matters here?

We could talk about retiring the MAP_LOCKED flag but I suspect that
would get significantly more pushback than adding a new mmap flag.

Likely that the overhead does not matter in most cases, but presumably
there are cases where it does (as we have a MAP_LOCKED flag today).
Even with the proposed new system calls I think we should have the
MAP_LOCKONFAULT for parity with MAP_LOCKED.

> 
> >- mlock() takes a `flags' argument.  Presently that's
> >   MLOCK_LOCKED|MLOCK_LOCKONFAULT.
> >
> >- munlock() takes a `flags' arument.  MLOCK_LOCKED|MLOCK_LOCKONFAULT
> >   to specify which flags are being cleared.
> >
> >- mlockall() and munlockall() ditto.
> >
> >
> >IOW, LOCKED and LOCKEDONFAULT are treated identically and independently.
> >
> >Now, that's how we would have designed all this on day one.  And I
> >think we can do this now, by adding new mlock2() and munlock2()
> >syscalls.  And we may as well deprecate the old mlock() and munlock(),
> >not that this matters much.
> >
> >*should* we do this?  I'm thinking "yes" - it's all pretty simple
> >boilerplate and wrappers and such, and it gets the interface correct,
> >and extensible.
> 
> If the new LOCKONFAULT functionality is indeed desired (I haven't
> still decided myself) then I agree that would be the cleanest way.

Do you disagree with the use cases I have listed or do you think there
is a better way of addressing those cases?

> 
> >What do others think?

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