Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT

[Eric B Munson]

On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote:

> On Mon, Aug 24, 2015 at 6:55 PM, Eric B Munson <emunson@xxxxxxxxxx> wrote:
> > On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote:
> >
> >> On Mon, Aug 24, 2015 at 6:09 PM, Eric B Munson <emunson@xxxxxxxxxx> wrote:
> >> > On Mon, 24 Aug 2015, Vlastimil Babka wrote:
> >> >
> >> >> On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote:
> >> >> >On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka <vbabka@xxxxxxx> wrote:
> >> >> >>On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote:
> >> >> >>>>
> >> >> >>>>
> >> >> >>>>I am in the middle of implementing lock on fault this way, but I cannot
> >> >> >>>>see how we will hanlde mremap of a lock on fault region.  Say we have
> >> >> >>>>the following:
> >> >> >>>>
> >> >> >>>>      addr = mmap(len, MAP_ANONYMOUS, ...);
> >> >> >>>>      mlock(addr, len, MLOCK_ONFAULT);
> >> >> >>>>      ...
> >> >> >>>>      mremap(addr, len, 2 * len, ...)
> >> >> >>>>
> >> >> >>>>There is no way for mremap to know that the area being remapped was lock
> >> >> >>>>on fault so it will be locked and prefaulted by remap.  How can we avoid
> >> >> >>>>this without tracking per vma if it was locked with lock or lock on
> >> >> >>>>fault?
> >> >> >>>
> >> >> >>>
> >> >> >>>remap can count filled ptes and prefault only completely populated areas.
> >> >> >>
> >> >> >>
> >> >> >>Does (and should) mremap really prefault non-present pages? Shouldn't it
> >> >> >>just prepare the page tables and that's it?
> >> >> >
> >> >> >As I see mremap prefaults pages when it extends mlocked area.
> >> >> >
> >> >> >Also quote from manpage
> >> >> >: If  the memory segment specified by old_address and old_size is locked
> >> >> >: (using mlock(2) or similar), then this lock is maintained when the segment is
> >> >> >: resized and/or relocated.  As a  consequence, the amount of memory locked
> >> >> >: by the process may change.
> >> >>
> >> >> Oh, right... Well that looks like a convincing argument for having a
> >> >> sticky VM_LOCKONFAULT after all. Having mremap guess by scanning
> >> >> existing pte's would slow it down, and be unreliable (was the area
> >> >> completely populated because MLOCK_ONFAULT was not used or because
> >> >> the process aulted it already? Was it not populated because
> >> >> MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to
> >> >> populate it all?).
> >> >
> >> > Given this, I am going to stop working in v8 and leave the vma flag in
> >> > place.
> >> >
> >> >>
> >> >> The only sane alternative is to populate always for mremap() of
> >> >> VM_LOCKED areas, and document this loss of MLOCK_ONFAULT information
> >> >> as a limitation of mlock2(MLOCK_ONFAULT). Which might or might not
> >> >> be enough for Eric's usecase, but it's somewhat ugly.
> >> >>
> >> >
> >> > I don't think that this is the right solution, I would be really
> >> > surprised as a user if an area I locked with MLOCK_ONFAULT was then
> >> > fully locked and prepopulated after mremap().
> >>
> >> If mremap is the only problem then we can add opposite flag for it:
> >>
> >> "MREMAP_NOPOPULATE"
> >> - do not populate new segment of locked areas
> >> - do not copy normal areas if possible (anonymous/special must be copied)
> >>
> >> addr = mmap(len, MAP_ANONYMOUS, ...);
> >> mlock(addr, len, MLOCK_ONFAULT);
> >> ...
> >> addr2 = mremap(addr, len, 2 * len, MREMAP_NOPOPULATE);
> >> ...
> >>
> >
> > But with this, the user must remember what areas are locked with
> > MLOCK_LOCKONFAULT and which are locked the with prepopulate so the
> > correct mremap flags can be used.
> >
> 
> Yep. Shouldn't be hard. You anyway have to do some changes in user-space.
> 

Sorry if I wasn't clear enough in my last reply, I think forcing
userspace to track this is the wrong choice.  The VM system is
responsible for tracking these attributes and should continue to be.

> 
> Much simpler for users-pace solution is a mm-wide flag which turns all further
> mlocks and MAP_LOCKED into lock-on-fault. Something like
> mlockall(MCL_NOPOPULATE_LOCKED).

This set certainly adds the foundation for such a change if you think it
would be useful.  That particular behavior was not part of my inital use
case though.

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