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.
