Re: Resurrecting the VM_PINNED discussion

[Eric B Munson <emunson@akamai.com>]

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On Tue, 03 Mar 2015, Vlastimil Babka wrote:

> 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?
> 

I spent quite some time looking for it as well, it is in vmscan.c

int __isolate_lru_page(struct page *page, isolate_mode_t mode)
{
...
        /* Compaction should not handle unevictable pages but CMA can do so */
        if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE))
                return ret;
...



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