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    SubjectRe: Resurrecting the VM_PINNED discussion
    On 03/03/2015 07:45 PM, Eric B Munson wrote:
    > 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.

    At least in the VM_PINNED thread after last lsf/mm, I don't see this mentioned.
    I found no references to mlocking in compaction.c, and in migrate.c there's just
    mlock_migrate_page() with comment:

    /*
    * mlock_migrate_page - called only from migrate_page_copy() to
    * migrate the Mlocked page flag; update statistics.
    */

    It also passes TTU_IGNORE_MLOCK to try_to_unmap(). So what am I missing? Where
    is this restriction?



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