Hi Peter,

On 2020/3/30 20:16, Peter Zijlstra wrote:

On Sat, Mar 28, 2020 at 12:30:50PM +0800, Zhenyu Ye wrote:

quoted

Hi all,

commit a6d60245 "Track which levels of the page tables have been cleared"
added cleared_(ptes|pmds|puds|p4ds) in struct mmu_gather, and the values
of them are set in some places. For example:

In include/asm-generic/tlb.h, pte_free_tlb() set the tlb->cleared_pmds:
---8<---
#ifndef pte_free_tlb
#define pte_free_tlb(tlb, ptep, address)			\
	do {							\
		__tlb_adjust_range(tlb, address, PAGE_SIZE);	\
		tlb->freed_tables = 1;				\
		tlb->cleared_pmds = 1;				\
		__pte_free_tlb(tlb, ptep, address);		\
	} while (0)
#endif
---8<---


However, in arch/s390/include/asm/tlb.h, pte_free_tlb() set the tlb->cleared_ptes:
---8<---
static inline void pte_free_tlb(struct mmu_gather *tlb, pgtable_t pte,
                                unsigned long address)
{
	__tlb_adjust_range(tlb, address, PAGE_SIZE);
	tlb->mm->context.flush_mm = 1;
	tlb->freed_tables = 1;
	tlb->cleared_ptes = 1;
	/*
	 * page_table_free_rcu takes care of the allocation bit masks
	 * of the 2K table fragments in the 4K page table page,
	 * then calls tlb_remove_table.
	 */
	page_table_free_rcu(tlb, (unsigned long *) pte, address);
}
---8<---


In my view, the cleared_(ptes|pmds|puds) and (pte|pmd|pud)_free_tlb
correspond one-to-one.  So we should set cleared_ptes in pte_free_tlb(),
then use it when needed.

So pte_free_tlb() clears a table of PTE entries, or a PMD level entity,
also see free_pte_range(). So the generic code makes sense to me. The
PTE level invalidations will have happened on tlb_remove_tlb_entry().

Thanks for your explanation. I can understand now.

quoted

I'm very confused about this. Which is wrong? Or is there something
I understand wrong?

I agree the s390 case is puzzling, Martin does s390 need a PTE level
invalidate for removing a PTE table or was this a mistake?

Then we should wait for @ Martin's reply.  Though s390 has never used
this value, I think we still should correct it if this is a mistake.

Thanks,
Zhenyu



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