** Changed in: ubuntu-z-systems
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/1904884
Title:
s390: dbginfo.sh triggers kernel panic, reading from
/sys/kernel/mm/page_idle/bitmap
Status in Ubuntu on IBM z Systems:
Fix Committed
Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Bionic:
Fix Committed
Status in linux source package in Focal:
Fix Committed
Status in linux source package in Groovy:
Fix Released
Status in linux source package in Hirsute:
Fix Released
Bug description:
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).
__________
System hangs on dbginfo.sh script execution.
Solution:
Commit 92fb1db26eef ("mm/page_idle.c: skip offline pages")
Included upstream since kernel v5.8, so it is already included in
Ubuntu 20.10, but not in 20.04 and earlier.
Commit 92fb1db26eef ("mm/page_idle.c: skip offline pages") applies
cleanly on ubuntu-focal, but not on ubuntu-bionic.
Adjustment / backport for bionic should be trivial, but it is not IBM
code and therefore the backport will be requested here by Canonical.
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