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

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Message-ID: <fbb00ac0-9104-8d25-f225-7b3d1b17a01f@huawei.com>
Date:   Sat, 28 Mar 2020 12:30:50 +0800
From:   Zhenyu Ye <yezhenyu2@...wei.com>
To:     Peter Zijlstra <peterz@...radead.org>, <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>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <arm@...nel.org>, <xiexiangyou@...wei.com>, <yezhenyu2@...wei.com>
Subject: [RFC][Qusetion] the value of cleared_(ptes|pmds|puds|p4ds) in struct
 mmu_gather

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.

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


Thanks,
Zhenyu