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

[Vlastimil Babka]

On 08/25/2015 03:41 PM, Michal Hocko wrote:
> On Fri 21-08-15 14:31:32, Eric B Munson 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?
>
> Yes mremap is a problem and it is very much similar to mmap(MAP_LOCKED).
> It doesn't guarantee the full mlock semantic because it leaves partially
> populated ranges behind without reporting any error.

Hm, that's right.

> Considering the current behavior I do not thing it would be terrible
> thing to do what Konstantin was suggesting and populate only the full
> ranges in a best effort mode (it is done so anyway) and document the
> behavior properly.
> "
>         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.
>
>         If the range is already fully populated and the range is
>         enlarged the new range is attempted to be fully populated
>         as well to preserve the full mlock semantic but there is no
>         guarantee this will succeed. Partially populated (e.g. created by
>         mlock(MLOCK_ONFAULT)) ranges do not have the full mlock semantic
>         so they are not populated on resize.
> "
>
> So what we have as a result is that partially populated ranges are
> preserved and fully populated ones work in the best effort mode the same
> way as they are now.
>
> Does that sound at least remotely reasonably?

I'll basically repeat what I said earlier:

- mremap scanning existing pte's to figure out the population would slow 
it down for no good reason
- it would be unreliable anyway:
  - example: was the area completely populated because MLOCK_ONFAULT 
was not used or because the  process faulted it already
  - example: was the area not completely populated because 
MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to populate 
it fully?

I think the first point is a pointless regression for workloads that use 
just plain mlock() and don't want the onfault semantics. Unless there's 
some shortcut? Does vma have a counter of how much is populated? (I 
don't think so?)
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