The number of scanned pages can be lower than the number of isolated pages
when isolating mirgratable or free pageblock.  The metric is being
reported in trace event and also used in vmstat.

some example output from trace where it shows nr_taken can be greater
than nr_scanned:

Produced by kernel v5.19-rc6
kcompactd0-42      [001] .....  1210.268022: mm_compaction_isolate_migratepages: range=(0x107ae4 ~ 0x107c00) nr_scanned=265 nr_taken=255
[...]
kcompactd0-42      [001] .....  1210.268382: mm_compaction_isolate_freepages: range=(0x215800 ~ 0x215a00) nr_scanned=13 nr_taken=128
kcompactd0-42      [001] .....  1210.268383: mm_compaction_isolate_freepages: range=(0x215600 ~ 0x215680) nr_scanned=1 nr_taken=128

mm_compaction_isolate_migratepages does not seem to have this
behaviour, but for the reason of consistency, nr_scanned should also be
taken care of in that side.

This behaviour is confusing since currently the count for isolated pages
takes account of compound page but not for the case of scanned pages.  And
given that the number of isolated pages(nr_taken) reported in
mm_compaction_isolate_template trace event is on a single-page basis, the
ambiguity when reporting the number of scanned pages can be removed by
also including compound page count.

Link: https://lkml.kernel.org/r/20220711202806.22296-1-william.lam@bytedance.comSigned-off-by: William Lam <william.lam@bytedance.com>
Reviewed-by: Punit Agrawal <punit.agrawal@bytedance.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>