Re: Resurrecting the VM_PINNED discussion

Davidlohr Bueso
2015-03-03T19:14:14Z

On Tue, 2015-03-03 at 19:35 +0100, Vlastimil Babka wrote:
> On 03/03/2015 06:41 PM, Eric B Munson wrote:> All,
> >
> > After LSF/MM last year Peter revived a patch set that would create
> > infrastructure for pinning pages as opposed to simply locking them.
> > AFAICT, there was no objection to the set, it just needed some help
> > from the IB folks.
> >
> > Am I missing something about why it was never merged? I ask because
> > Akamai has bumped into the disconnect between the mlock manpage,
> > Documentation/vm/unevictable-lru.txt, and reality WRT compaction and
> > locking. A group working in userspace read those sources and wrote a
> > tool that mmaps many files read only and locked, munmapping them when
> > they are no longer needed. Locking is used because they cannot afford a
> > major fault, but they are fine with minor faults. This tends to
> > fragment memory badly so when they started looking into using hugetlbfs
> > (or anything requiring order > 0 allocations) they found they were not
> > able to allocate the memory. They were confused based on the referenced
> > documentation as to why compaction would continually fail to yield
> > appropriately sized contiguous areas when there was more than enough
> > free memory.
>
> So you are saying that mlocking (VM_LOCKED) prevents migration and thus
> compaction to do its job? If that's true, I think it's a bug as it is AFAIK
> supposed to work just fine.
>
> > I would like to see the situation with VM_LOCKED cleared up, ideally the
> > documentation would remain and reality adjusted to match and I think
> > Peter's VM_PINNED set goes in the right direction for this goal. What
> > is missing and how can I help?
>
> I don't think VM_PINNED would help you. In fact it is VM_PINNED that improves
> accounting for the kind of locking (pinning) that *does* prevent page migration
> (unlike mlocking)... quoting the patchset cover letter:
>
> "These patches introduce VM_PINNED infrastructure, vma tracking of persistent
> 'pinned' page ranges. Pinned is anything that has a fixed phys address (as
> required for say IO DMA engines) and thus cannot use the weaker VM_LOCKED. One
> popular way to pin pages is through get_user_pages() but that not nessecarily
> the only way."

Yeah, this also makes it pretty clear:

""
Firstly, various subsystems (perf, IB amongst others) 'pin'
significant chunks of memory (through holding page refs or custom
maps), because this memory is unevictable we must test this against
RLIMIT_MEMLOCK.

...

Thirdly, because VM_LOCKED does allow unmapping (and therefore page
migration) the -rt people are not pleased and would very much like
something stronger.
""