From 57834ce5a6a47df282c8419019ba5495eac58fb9 Mon Sep 17 00:00:00 2001 From: Gerald Schaefer Date: Thu, 21 Aug 2025 19:00:03 +0200 Subject: [PATCH] s390/mm: Prevent possible preempt_count overflow The s390 implementation of ptep_modify_prot_start() currently does preempt_disable(), and the preempt_enable() is done later in ptep_modify_prot_commit(). This logic is not really required, because the PTE lock must be held over the complete prot_start/commit transaction, as described in the comment of the generic implementation of ptep_modify_prot_start(). That comment also mentions that this interface should be batchable, and modify_prot_start_ptes() might start a transaction over a batch of PTEs, implemented as a simple loop over ptep_modify_prot_start(). In this case, the preempt_disable() in ptep_modify_prot_start() would be called multiple times, before the corresponding preempt_enable() calls happen, and this can lead to a preempt_count overflow. To fix this, simply remove the preempt_disable/enable() calls in ptep_modify_prot_start/commit(), and rely on the PTE lock being held. Commit cac1db8c3aad ("mm: optimize mprotect() by PTE batching") made use of this PTE batching for the first time, and triggers warnings like this: DEBUG_LOCKS_WARN_ON((preempt_count() & PREEMPT_MASK) >= PREEMPT_MASK - 10) BUG: sleeping function called from invalid context at mm/mprotect.c:576 Hence, add a Fixes tag on that commit. Not because it is broken, but to make sure that it won't get backported w/o also this fix for s390. Fixes: cac1db8c3aad ("mm: optimize mprotect() by PTE batching") Reviewed-by: Alexander Gordeev Signed-off-by: Gerald Schaefer Signed-off-by: Alexander Gordeev --- arch/s390/mm/pgtable.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c index 60688be4e876..50eb57c976bc 100644 --- a/arch/s390/mm/pgtable.c +++ b/arch/s390/mm/pgtable.c @@ -335,7 +335,6 @@ pte_t ptep_modify_prot_start(struct vm_area_struct *vma, unsigned long addr, int nodat; struct mm_struct *mm = vma->vm_mm; - preempt_disable(); pgste = ptep_xchg_start(mm, addr, ptep); nodat = !!(pgste_val(pgste) & _PGSTE_GPS_NODAT); old = ptep_flush_lazy(mm, addr, ptep, nodat); @@ -360,7 +359,6 @@ void ptep_modify_prot_commit(struct vm_area_struct *vma, unsigned long addr, } else { set_pte(ptep, pte); } - preempt_enable(); } static inline void pmdp_idte_local(struct mm_struct *mm, -- 2.47.3