Re: [RFC v4 PATCH 2/5] mm/__free_one_page: skip merge for order-0 page unless compaction failed

[Daniel Jordan]

On Fri, Oct 19, 2018 at 09:54:35AM +0100, Mel Gorman wrote:
> On Fri, Oct 19, 2018 at 01:57:03PM +0800, Aaron Lu wrote:
> > > 
> > > I don't think this is the right way of thinking about it because it's
> > > possible to have the system split in such a way so that the migration
> > > scanner only encounters unmovable pages before it meets the free scanner
> > > where unmerged buddies were in the higher portion of the address space.
> > 
> > Yes it is possible unmerged pages are in the higher portion.
> > 
> > My understanding is, when the two scanners meet, all unmerged pages will
> > be either used by the free scanner as migrate targets or sent to merge
> > by the migration scanner.
> > 
> 
> It's not guaranteed if the lower portion of the address space consisted
> entirely of pages that cannot migrate (because they are unmovable or because
> migration failed due to pins). It's actually a fundamental limitation
> of compaction that it can miss migration and compaction opportunities
> due to how the scanners are implemented. It was designed that way to
> avoid pageblocks being migrated unnecessarily back and forth but the
> downside is missed opportunities.
> 
> > > You either need to keep unmerged buddies on a separate list or search
> > > the order-0 free list for merge candidates prior to compaction.
> > > 
> > > > > It's needed to form them efficiently but excessive reclaim or writing 3
> > > > > to drop_caches can also do it. Be careful of tying lazy buddy too
> > > > > closely to compaction.
> > > > 
> > > > That's the current design of this patchset, do you see any immediate
> > > > problem of this? Is it that you are worried about high-order allocation
> > > > success rate using this design?
> > > 
> > > I've pointed out what I see are the design flaws but yes, in general, I'm
> > > worried about the high order allocation success rate using this design,
> > > the reliance on compaction and the fact that the primary motivation is
> > > when THP is disabled.
> > 
> > When THP is in use, zone lock contention is pretty much nowhere :-)
> > 
> > I'll see what I can get with 'address space range' lock first and will
> > come back to 'lazy buddy' if it doesn't work out.

With the address space range idea, wouldn't the zone free_area require changes
too?  I can't see how locking by address range could synchronize it as it
exists now otherwise, with per order/mt list heads.

One idea is to further subdivide the free area according to how the locking
works and find some reasonable way to handle having to search for pages of a
given order/mt in multiple places.