[PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT
vbabka at suse.cz
Tue Aug 25 06:55:46 PDT 2015
On 08/25/2015 03:41 PM, Michal Hocko 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
> 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.
Hm, that's right.
> 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?
I'll basically repeat what I said earlier:
- mremap scanning existing pte's to figure out the population would slow
it down for no good reason
- it would be unreliable anyway:
- example: was the area completely populated because MLOCK_ONFAULT
was not used or because the process faulted it already
- example: was the area not completely populated because
MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to populate
I think the first point is a pointless regression for workloads that use
just plain mlock() and don't want the onfault semantics. Unless there's
some shortcut? Does vma have a counter of how much is populated? (I
don't think so?)
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