Re: [PATCH v6 06/38] mm: Add default definition of set_ptes()

[Matthew Wilcox <willy@infradead.org>]

On Thu, Oct 12, 2023 at 02:53:05PM +0100, David Woodhouse wrote:
> > +       arch_enter_lazy_mmu_mode();
> > +       for (;;) {
> > +               set_pte(ptep, pte);
> > +               if (--nr == 0)
> > +                       break;
> > +               ptep++;
> > +               pte = __pte(pte_val(pte) + (1UL << PFN_PTE_SHIFT));
> > +       }
> > +       arch_leave_lazy_mmu_mode();
> 
> This breaks the Xen PV guest.
> 
> In move_ptes() in mm/mremap.c we arch_enter_lazy_mmu_mode() and then
> loop calling set_pte_at(). Which now (or at least in a few commits time
> when you wire it up for x86 in commit a3e1c9372c9b959) ends up in your
> implementation of set_ptes(), calls arch_enter_lazy_mmu_mode() again,
> and:
> 
> [    0.628700] ------------[ cut here ]------------
> [    0.628718] kernel BUG at arch/x86/kernel/paravirt.c:144!

Easy fix ... don't do that ;-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index af7639c3b0a3..f3da8836f689 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -231,9 +231,11 @@ static inline pte_t pte_next_pfn(pte_t pte)
 static inline void set_ptes(struct mm_struct *mm, unsigned long addr,
 		pte_t *ptep, pte_t pte, unsigned int nr)
 {
+	bool multiple = nr > 1;
 	page_table_check_ptes_set(mm, ptep, pte, nr);
 
-	arch_enter_lazy_mmu_mode();
+	if (multiple)
+		arch_enter_lazy_mmu_mode();
 	for (;;) {
 		set_pte(ptep, pte);
 		if (--nr == 0)
@@ -241,7 +243,8 @@ static inline void set_ptes(struct mm_struct *mm, unsigned long addr,
 		ptep++;
 		pte = pte_next_pfn(pte);
 	}
-	arch_leave_lazy_mmu_mode();
+	if (multiple)
+		arch_leave_lazy_mmu_mode();
 }
 #endif
 #define set_pte_at(mm, addr, ptep, pte) set_ptes(mm, addr, ptep, pte, 1)

I think long-term, we should make lazy_mmu_mode nestable.  But this is
a reasonable quick fix.