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s390/mm: Remove possible false-positive warning in pte_free_defer()

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commit 5647f61ad9 upstream.

Commit 8211dad627 ("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: 8211dad627 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Gerald Schaefer 2025-07-09 20:34:30 +02:00 committed by Greg Kroah-Hartman
parent c7c87046b4
commit db89c0e0f6
1 changed files with 0 additions and 5 deletions
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5
arch/s390/mm/pgalloc.c
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@ -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 */