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

[Konstantin Khlebnikov <koct9i@gmail.com>]

On Mon, Aug 24, 2015 at 6:09 PM, Eric B Munson <emunson@akamai.com> 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@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().

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);
...

>
>> >>
>> >>>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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