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 <vba...@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 <rient...@google.com>
