linux-kernel - Re: [RFC][Qusetion] the value of cleared_(ptes|pmds|puds|p4ds) in struct mmu

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Date:   Tue, 14 Apr 2020 09:05:56 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Zhenyu Ye <yezhenyu2@...wei.com>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>
Cc:     npiggin@...il.com, will.deacon@....com, mingo@...nel.org,
        torvalds@...ux-foundation.org, schwidefsky@...ibm.com,
        akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        Marc Zyngier <maz@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, arm@...nel.org, xiexiangyou@...wei.com
Subject: Re: [RFC][Qusetion] the value of cleared_(ptes|pmds|puds|p4ds) in
 struct mmu_gather

Gerald,

can you have a look?

On 30.03.20 14:16, Peter Zijlstra wrote:
> On Sat, Mar 28, 2020 at 12:30:50PM +0800, Zhenyu Ye wrote:
>> 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().
> 
>> 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?
>