On Tue, 03 Mar 2015, 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.
Agreed. But as has been discussed in the threads around the VM_PINNED work, there are people that are relying on the fact that VM_LOCKED promises no minor faults. Which is why the behavoir has remained. > > > 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: VM_PINNED itself doesn't help us, but it would allow us to make VM_LOCKED use only the weaker 'no major fault' semantics while still providing a way for anyone that needs the stronger 'no minor fault' promise to get the semantics they need. > > "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 > > >
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