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

[Vlastimil Babka]

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?

> There might be a problem after failed populate: remap will handle them
> as lock on fault. In this case we can fill ptes with swap-like non-present
> entries to remember that fact and count them as should-be-locked pages.

I don't think we should strive to have mremap try to fix the inherent 
unreliability of mmap (MAP_POPULATE)?
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