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

Commit 8211dad ("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: 8211dad ("s390: add pte_free_defer() for pgtables sharing page")
Cc: <[email protected]> # 6.6+
Reviewed-by: Alexander Gordeev <[email protected]>
Reviewed-by: Claudio Imbrenda <[email protected]>
Signed-off-by: Gerald Schaefer <[email protected]>
Signed-off-by: Alexander Gordeev <[email protected]>

Author: gerald-schaefer

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 */