On Thu, Dec 03, 2020 at 05:36:07PM -0500, Johannes Weiner wrote:

Correct, it's only interesting for pages under LRU management - page
cache and swap pages. It should not matter for direct IO.

The VM uses the page flag to tell the difference between cold faults
(empty cache startup e.g.), and thrashing pages which are being read
back not long after they have been reclaimed. This influences reclaim
behavior, but can also indicate a general lack of memory.

I really wonder if we should move setting the flag out of bio_add_page
and into the writeback code, as it will do the wrong things for
non-writeback I/O, that is direct I/O or its in-kernel equivalents.