Re: Resurrecting the VM_PINNED discussion

[Vlastimil Babka <vbabka@suse.cz>]

On 03/03/2015 10:52 PM, Eric B Munson wrote:
> 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:
>> > > 
>> > > 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.

No, you were correct and thanks for the hint. It's only ISOLATE_UNEVICTABLE from
isolate_migratepages_range(), which is CMA, not regular compaction.
But I wonder, can we change this even after VM_PINNED is introduced, if existing
code depends on "no minor faults in mlocked areas", whatever the docs say? On
the other hand, compaction is not the only source of migrations. I wonder what
the NUMA balancing does (not) about mlocked areas...

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