From: SeongJae Park <redacted>

On Mon, 14 Jun 2021 19:04:20 -0700 Andrew Morton [off-list ref] wrote:

On Mon, 14 Jun 2021 13:49:26 +0000 SeongJae Park [off-list ref] wrote:

quoted

From: SeongJae Park <redacted>

Hello Matthew,

On Sat, 12 Jun 2021 01:07:14 +0100 "Matthew Wilcox (Oracle)" [off-list ref] wrote:

quoted

In discussion with other MM developers around how idle page tracking
should be fixed for transparent huge pages, several expressed the opinion
that it should be removed as it is inefficient at accomplishing the
job that it is supposed to, and we have better mechanisms (eg uffd) for
accomplishing the same goals these days.

I think the THP case[1] is an intended behavior[2].  Could you please share a
link to the discussion or a detailed summary if possible?

quoted

Mark the feature as BROKEN for now and we can remove it entirely in a
few months if nobody complains.  It is not enabled by Android, ChromeOS,
Debian, Fedora or SUSE.  Red Hat enabled it with RHEL-8.1 and UEK followed
suit, but I have been unable to find why RHEL enabled it.

Amazon Linux is also using it[3], for DAMON[4].  In detail, DAMON doesn't use
Idle Page Tracking but PG_Idle in kernel space, to avoid interfering the
reclaim logic[5].  So, I'm ok with removing the Idle Page Tracking user space
interface, but gonna be opposed to removing PG_Idle.

Nevertheless, the interference is not a real problem to DAMON, as DAMON is
aimed to provide just a reasonable quality of the monitoring, rather than
strict correctness.  Hence, if people think the interference is also not a
problem for the reclaim logic (after all, it does nothing unless sysadmin
manually turns it on in runtime, and can be turned off at anytime), I would
simply update DAMON code to don't use PG_Idle, add warnings in the doc, and
wouldn't be opposed to this change.

Couldn't the DAMON patchset simply re-add PG_Idle?  Perhaps with a new
name which is more appropriate to the DAMON usage?

Good point, that makes sense.  Thank you, Andrew.

Nevertheless, I still not fully understand why Idle Page Tracking is considered
broken, and therefore worrying if I will end up reintroducing PG_Idle with
DAMON patchset in the broken form.  It would still be great if a link to the
dicussion could be provided.


Thanks,
SeongJae Park