On Tue, 03 Mar 2015, Eric B Munson wrote:
> 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;
> ...
>
>
And that demonstrates that I haven't spent enough time with this code,
that isn't the restriction because when this is called from compaction.c
the mode is set to ISOLATE_UNEVICTABLE. So back to reading the code.
Eric
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