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 > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>