Re: Resurrecting the VM_PINNED discussion

Vlastimil Babka
2015-03-03T18:35:18Z

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."

> Thanks,
> Eric