Re: [PATCH 1/3] mm, compaction: extend pageblock_skip_persistent() to all compound pages

[David Rientjes <rientjes@google.com>]

On Thu, 2 Nov 2017, Vlastimil Babka wrote:

> The pageblock_skip_persistent() function checks for HugeTLB pages of pageblock
> order. When clearing pageblock skip bits for compaction, the bits are not
> cleared for such pageblocks, because they cannot contain base pages suitable
> for migration, nor free pages to use as migration targets.
> 
> This optimization can be simply extended to all compound pages of order equal
> or larger than pageblock order, because migrating such pages (if they support
> it) cannot help sub-pageblock fragmentation. This includes THP's and also
> gigantic HugeTLB pages, which the current implementation doesn't persistently
> skip due to a strict pageblock_order equality check and not recognizing tail
> pages.
> 
> While THP pages are generally less "persistent" than HugeTLB, we can still
> expect that if a THP exists at the point of __reset_isolation_suitable(), it
> will exist also during the subsequent compaction run. The time difference here
> could be actually smaller than between a compaction run that sets a
> (non-persistent) skip bit on a THP, and the next compaction run that observes
> it.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

This was originally done only for hugetlb because it was considered to be 
more persistent than thp, but with the presence of the deferred split 
shrinker I think this is fine.  It might be helpful to mention that the 
compound page is not synchronously split like it was prior to the thp 
refcounting patchset, however.

Acked-by: David Rientjes <rientjes@google.com>