Re: [PATCH] mm/page_alloc: use ac->high_zoneidx for classzone_idx

[Mel Gorman]

On Wed, May 16, 2018 at 11:35:55AM +0200, Vlastimil Babka wrote:
> On 05/08/2018 03:00 AM, Joonsoo Kim wrote:
> >> classzone predates my involvement with Linux but I would be less concerneed
> >> about what the original intent was and instead ensure that classzone index
> >> is consistent, sane and potentially renamed while preserving the intent of
> >> "reserve pages in lower zones when an allocation request can use higher
> >> zones". While historically the critical intent was to preserve Normal and
> >> to a lesser extent DMA on 32-bit systems, there still should be some care
> >> of DMA32 so we should not lose that.
> > 
> > Agreed!
> > 
> >> With the patch, the allocator looks like it would be fine as just
> >> reservations change. I think it's unlikely that CMA usage will result
> >> in lowmem starvation.  Compaction becomes a bit weird as classzone index
> >> has no special meaning versis highmem and I think it'll be very easy to
> >> forget.
> 
> I don't understand this point, what do you mean about highmem here?

I mean it has no special meaning as compaction is not primarily concerned
with lowmem protections as it compacts within a zone. It preserves watermarks
but it does not have the same degree of criticality as the page allocator
and reclaim is concerned with.

> I've
> checked and compaction seems to use classzone_idx 1) to pass it to
> watermark checks as part of compaction suitability checks, i.e. the
> usual lowmem protection, and 2) to limit compaction of higher zones in
> kcompactd if the direct compactor can't use them anyway - seems this
> part has currently the same zone imbalance problem as reclaim.
> 

Originally the watermark check for compaction was primarily about not
depleting a single zone but the checks were duplicated anyway. It's not
actually super critical for it to preserve lowmem zones as any memory
usage by compaction is transient.

> > Agreed!
> > I will update this patch to reflect your comment. If someone have an idea
> > on renaming this variable, please let me know.
> 
> Pehaps max_zone_idx? Seems a bit more clear than "high_zoneidx". And I
> have no idea what was actually meant by "class".
> 

I don't have a better suggestion.

-- 
Mel Gorman
SUSE Labs