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

[Konstantin Khlebnikov]

On Tue, Aug 25, 2015 at 4:41 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Fri 21-08-15 14:31:32, Eric B Munson 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?
>
> Yes mremap is a problem and it is very much similar to mmap(MAP_LOCKED).
> It doesn't guarantee the full mlock semantic because it leaves partially
> populated ranges behind without reporting any error.
>
> Considering the current behavior I do not thing it would be terrible
> thing to do what Konstantin was suggesting and populate only the full
> ranges in a best effort mode (it is done so anyway) and document the
> behavior properly.
> "
>        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.
>
>        If the range is already fully populated and the range is
>        enlarged the new range is attempted to be fully populated
>        as well to preserve the full mlock semantic but there is no
>        guarantee this will succeed. Partially populated (e.g. created by
>        mlock(MLOCK_ONFAULT)) ranges do not have the full mlock semantic
>        so they are not populated on resize.
> "
>
> So what we have as a result is that partially populated ranges are
> preserved and fully populated ones work in the best effort mode the same
> way as they are now.
>
> Does that sound at least remotely reasonably?

The problem is that mremap have to scan ptes to detect that and old behaviour
becomes very fragile: one fail and mremap will never populate that vma again.
For now I think new flag "MREMAP_NOPOPULATE" is a better option.

>
>
> --
> Michal Hocko
> SUSE Labs
>
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