I happened to be going through the source code for write_cache_pages(),
and I came across a reference to AOP_WRITEPAGE_ACTIVATE. I was curious
what the heck that was, so I did search for it, and found this in
Documentation/filesystems/vfs.txt:
If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
try too hard if there are problems, and may choose to write out
other pages from the mapping if that is easier (e.g. due to
internal dependencies). If it chooses ...
