[SRU][B][F] s390: dbginfo.sh triggers kernel panic, reading from /sys/kernel/mm/page_idle/bitmap (LP: 1904884)

[frank.heimes at canonical.com]

BugLink: https://bugs.launchpad.net/bugs/1904884

SRU Justification:

[Impact]

* While executing dbginfo.sh (a script to collect runtime, configuration, and trace information on s390x) the systems hangs.

* This is because 'idle page tracking' users can pass random pfn, that might be mapped to
an offline page - and attempts to access offline pages lead to the hang.

* It needs to be avoided that such pages are accessed.

* The upstream commit modifies 'page_idle_get_page()' to use 'pfn_to_online_page()' instead of a
'pfn_valid()' and 'pfn_to_page()' combination, so that the pfn mapped to an offline page is skipped.

[Fix]

* 92fb1db26eef "mm/page_idle.c: skip offline pages"

[Test Case]

* IBM Z or LinuxONE hardware with Ubuntu Server 18.04 (GA kernel, 4.15) installed.

* Execute a test application that tries to access offline pages.

* Or execute dbginfo.sh with having some offline (idle) pages in the system.

[Regression Potential]

* There is a certain regression risk, especially for bionic, since the structure in the kernel 4.15 is a bit different compared to kernel 5.4 (and newer).

* However, for newer kernels the modification is pretty save, since it's upstream accepted since kernel 5.8 and with that already inluded in hirsute and groovy.

* And the patch is fine (and cherry picks cleanly) for focal as well.

* For bionic there is a slightly conflicting context, since the struct 'zone' was replaced by 'pg_data_t *pgdat' (by another commit: 92fb1db26eef),
  but that change (or any change to the struct zone) would not be necessary to fix the uninitialized struct page access.

* Hence the upstream commit/patch needs to be adjusted/backported to bionic 4.15, largely by replacing line 'pg_data_t *pgdat;' with 'struct zone *zone;' (or actually leaving this line).

* But this needs to be carefully considered, since the handling of idle pages could be harmful, in the end it could make things even worse, means break even more.

[Other]

* The patch got upstream accepted with kernel v5.8, hence it's already is in groovy and hirsute.

* The upstream commit cherry picks cleanly to focal, but for bionic a backport is needed.

* Hence this kernel SRU request is for focal (cherry-pick) and bionic (backport).

SeongJae Park (1):
  mm/page_idle.c: skip offline pages

 mm/page_idle.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

-- 
2.25.1