The patch below does not apply to the 6.6-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y git checkout FETCH_HEAD git cherry-pick -x 5647f61ad9171e8f025558ed6dc5702c56a33ba3 # <resolve conflicts, build, test, etc.> git commit -s git send-email --to 'stable@vger.kernel.org' --in-reply-to '2025081255-shabby-impound-4a47@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 5647f61ad9171e8f025558ed6dc5702c56a33ba3 Mon Sep 17 00:00:00 2001 From: Gerald Schaefer gerald.schaefer@linux.ibm.com Date: Wed, 9 Jul 2025 20:34:30 +0200 Subject: [PATCH] s390/mm: Remove possible false-positive warning in pte_free_defer()

Commit 8211dad627981 ("s390: add pte_free_defer() for pgtables sharing page") added a warning to pte_free_defer(), on our request. It was meant to warn if this would ever be reached for KVM guest mappings, because the page table would be freed w/o a gmap_unlink(). THP mappings are not allowed for KVM guests on s390, so this should never happen.

However, it is possible that the warning is triggered in a valid case as false-positive.

s390_enable_sie() takes the mmap_lock, marks all VMAs as VM_NOHUGEPAGE and splits possibly existing THP guest mappings. mm->context.has_pgste is set to 1 before that, to prevent races with the mm_has_pgste() check in MADV_HUGEPAGE.

khugepaged drops the mmap_lock for file mappings and might run in parallel, before a vma is marked VM_NOHUGEPAGE, but after mm->context.has_pgste was set to 1. If it finds file mappings to collapse, it will eventually call pte_free_defer(). This will trigger the warning, but it is a valid case because gmap is not yet set up, and the THP mappings will be split again.

Therefore, remove the warning and the comment.

Fixes: 8211dad627981 ("s390: add pte_free_defer() for pgtables sharing page") Cc: stable@vger.kernel.org # 6.6+ Reviewed-by: Alexander Gordeev agordeev@linux.ibm.com Reviewed-by: Claudio Imbrenda imbrenda@linux.ibm.com Signed-off-by: Gerald Schaefer gerald.schaefer@linux.ibm.com Signed-off-by: Alexander Gordeev agordeev@linux.ibm.com

diff --git a/arch/s390/mm/pgalloc.c b/arch/s390/mm/pgalloc.c index b449fd2605b0..d2f6f1f6d2fc 100644 --- a/arch/s390/mm/pgalloc.c +++ b/arch/s390/mm/pgalloc.c @@ -173,11 +173,6 @@ void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable) struct ptdesc *ptdesc = virt_to_ptdesc(pgtable);

call_rcu(&ptdesc->pt_rcu_head, pte_free_now); - /* - * THPs are not allowed for KVM guests. Warn if pgste ever reaches here. - * Turn to the generic pte_free_defer() version once gmap is removed. - */ - WARN_ON_ONCE(mm_has_pgste(mm)); } #endif /* CONFIG_TRANSPARENT_HUGEPAGE */