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

[Konstantin Khlebnikov]

On Mon, Aug 24, 2015 at 6:55 PM, Eric B Munson <emunson@xxxxxxxxxx> wrote:
> 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.
>

Yep. Shouldn't be hard. You anyway have to do some changes in user-space.


Much simpler for users-pace solution is a mm-wide flag which turns all further
mlocks and MAP_LOCKED into lock-on-fault. Something like
mlockall(MCL_NOPOPULATE_LOCKED).
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