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

[Konstantin Khlebnikov <koct9i@gmail.com>]

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.

>
>> 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.

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