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

[Eric B Munson <emunson@akamai.com>]

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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@suse.cz> 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().

> >>
> >>>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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