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

[Vlastimil Babka]

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?).

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.

> > > 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)?
>
> I don't think so. MAP_POPULATE works only when mmap happens.
> Flag MREMAP_POPULATE might be a good idea. Just for symmetry.

Maybe, but please do it as a separate series.
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