Re: Resurrecting the VM_PINNED discussion

Vlastimil Babka
2015-03-03T20:20:54Z

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?