I've seen this too.  Do we know which extension is doing this? 

Hi wvk,
  Are you sure it's disabled by some extension? Do you mean the failure_reason is always LIVE_STATE_EXTENSION_DISALLOWED but no other values? What do you see in about://discards ? 
 

In latest versions of Chrome OS, a PROTECTED_BACKGROUND_TAB can be discarded when there is memory pressure. The "protection" only means that it won't be discarded if discarding all non-protected tabs is not enough to exit the memory pressure state.

Can you provide the Chrome version and complete log when you observe this issue?

Sorry, it looks like I might've filed this bug unnecessarily. I took another look at chrome://discards and it looks like only the bookmarked tabs are protected by an extension. When I was manually creating memory pressure, I had launched all of the tabs from bookmarks, which is why I saw them all get ranked as PROTECTED_BACKGROUND_TAB.

You have an extension that protects bookmarked tabs? I don't think this is a functionality provided by Chrome.

Not that I'm aware of. These bookmarked tabs are ranked as PROTECTED_BACKGROUND_TAB with reason "Tab is protected by extension" even when no extensions are installed (according to chrome://extensions). 

"Tab has been protected by an extension" should only appear when an extension protects a tab using the autoDiscardable API (https://developer.chrome.com/extensions/tabs) or when you simulate this by clicking in the "Auto Discardable" column in chrome://discards. If you reproduce this, can you provide the exact steps + your Chrome version number? Thanks!

I did a little bit more digging, and it seems that it is not bookmarked tabs, but rather the crosh extension. I noticed that tabs become PROTECTED_BACKGROUND as soon as crosh is opened, and indeed, the code responsible is here: https://cs.corp.google.com/chromeos_public/src/third_party/libapps/nassh/js/nassh.js?rcl=dc00f9157f9ad498b627d952697ca8a23417ff2e&l=246

nassh.disableTabDiscarding() is called at window.onload, and all the tabs in that window have their autoDiscardable property set to false. I was seeing this behavior in my tests because crosh was one of my bookmarked tabs.
I'm unsure whether this is the extension's intended behavior, because in the original bug ( https://crbug.com/868155 ) it seems like they intended to prevent only the crosh tab itself from being discarded.

The steps I took to reproduce on caroline R73-11424.0.0:
1. Open a few tabs (e.g. reddit.com, nytimes.com, bbc.com)
2. Open chrome://discards and note the reasons under 'Can discard?' for each tab
3. Open crosh (Ctrl+Alt+T) in the same window
4. Open chrome://discards again, all tabs should now have 'Tab has been protected by an extension' in their 'Can discard?' section

wvk@: Thanks for investigating this.

vapier@: The crosh extension protects all tabs in the current window. Could you update the code to instead protect only tabs that match the extension's url? Relevant code is here: https://cs.corp.google.com/chromeos_public/src/third_party/libapps/nassh/js/nassh.js?rcl=dc00f9157f9ad498b627d952697ca8a23417ff2e&l=246

The following revision refers to this bug:
  https://chromium.googlesource.com/apps/libapps/+/469621dbc9010b73f46ae137f082baf1d35b42d5

commit 469621dbc9010b73f46ae137f082baf1d35b42d5
Author: Mike Frysinger <vapier@chromium.org>
Date: Thu Dec 20 19:39:33 2018

nassh: fix tab discarding to only affect current tab

When we're opened in a tab, chrome.tabs.query will return all tabs in
that window, so we'd end up disabling automatic tab discarding on all
the other (non-crosh/nassh) tabs.  Only look up the current tabid and
disable tab discarding on it.

Bug:  912786 
Change-Id: Id197abb56e2087b6f6c80830263fa3e6a6482fd0
Reviewed-on: https://chromium-review.googlesource.com/c/1387066
Reviewed-by: Vitaliy Shipitsyn <vsh@google.com>
Tested-by: Mike Frysinger <vapier@chromium.org>

[modify] https://crrev.com/469621dbc9010b73f46ae137f082baf1d35b42d5/nassh/js/nassh.js

Thanks a lot for fixing this quickly!