[Ubuntu-24.04] Hugepage memory is not getting released even after destroying the guest!

[bugproxy]

---Problem Description---

Hugepages memory is not getting released even after destroying the guest

Machine Type = P10 Denali LPAR

---uname output---
Linux ubuntu2404lp3 6.8.0-22-generic #22-Ubuntu SMP Thu Apr 4 22:47:57 UTC 2024 ppc64le ppc64le ppc64le GNU/Linux

---Steps to Reproduce---
1. Create a guest which is backed by hugepages.
2. Destroy the guest
3. execute "free -h" or "cat /proc/meminfo" to see that Hugepage memory is still getting held.

HugePages_Total: 20480
HugePages_Free: 20419
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
Hugetlb: 41943040 kB
DirectMap4k: 0 kB
DirectMap64k: 52428800 kB
DirectMap2M: 0 kB
DirectMap1G: 0 kB
root@ubuntu2404lp3:~# virsh list --all
 Id Name State
---------------------------
 - ramlp2g1 shut off
 - ramlp2g2 shut off
 - ramlp2g3 shut off
 - ramlp3g3 shut off

root@ubuntu2404lp3:~# free -h
               total used free shared buff/cache available
Mem: 48Gi 43Gi 4.6Gi 2.6Mi 277Mi 4.6Gi
Swap: 8.0Gi 243Mi 7.8Gi
root@ubuntu2404lp3:~#

====
This is an issue created by commit 1b151e2435fc ("block: Remove special-casing of compound pages") that moved the direct-io hugetlb handling from compound pages to folios.

Following commit has been proposed and merged into 6.9-rc1 which fixes this issue.
38b43539d64b2fa020b3b9a752a986769f87f7a6("block: Fix page refcounts for unaligned buffers in __bio_release_pages()")

So the same needs to be backported to the Ubuntu24.04 kernel as well.

[Frank Heimes (fheimes)]

I just had a look at noble's master-next tree:
$ git remote get-url origin
git://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/noble
$ git branch
* master-next

and found that commit 19c1ceeca8ed is already applied:
$ git log --oneline | grep -n "block: Fix page refcounts for unaligned buffers in __bio_release_pages"
300:19c1ceeca8ed block: Fix page refcounts for unaligned buffers in __bio_release_pages()

but not yet tagged with (incl. in) any version:
$ git tag --contains 19c1ceeca8ed
$

This is probably because it just came in via upstream stable updates:
$ git show 19c1ceeca8ed | sed '/^diff --git/q'
commit 19c1ceeca8ed31d207859e659482ee83ea1959f5
Author: Tony Battersby <email address hidden>
Date: Thu Feb 29 13:08:09 2024 -0500

    block: Fix page refcounts for unaligned buffers in __bio_release_pages()

    BugLink: https://bugs.launchpad.net/bugs/2060531

    [ Upstream commit 38b43539d64b2fa020b3b9a752a986769f87f7a6 ]

    Fix an incorrect number of pages being released for buffers that do not
    start at the beginning of a page.

    Fixes: 1b151e2435fc ("block: Remove special-casing of compound pages")
    Cc: <email address hidden>
    Signed-off-by: Tony Battersby <email address hidden>
    Tested-by: Greg Edwards <email address hidden>
    Link: https://<email address hidden>
    Signed-off-by: Jens Axboe <email address hidden>
    Signed-off-by: Sasha Levin <email address hidden>
    Signed-off-by: Paolo Pisati <email address hidden>

diff --git a/block/bio.c b/block/bio.c

And since the lastest tagged Ubuntu kernel version in master-next is: Ubuntu-6.8.0-31.31
$ git log --oneline | grep -n $(git describe --tags --abbrev=0)
1170:7fdb45c9bbbc (tag: Ubuntu-6.8.0-31.31) UBUNTU: Ubuntu-6.8.0-31.31

commit 19c1ceeca8ed will be automatically part of the next/upcoming Ubuntu kernel (> Ubuntu-6.8.0-31.31).

With that I'm updating the status of this bug to Fix Committed.