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 6:09 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.

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