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Date:   Sun, 3 Oct 2021 14:10:19 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Nadav Amit <nadav.amit@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>, Peter Xu <peterx@redhat.com>,
        Nadav Amit <namit@vmware.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Nick Piggin <npiggin@gmail.com>, x86@kernel.org
Subject: Re: [PATCH 1/2] mm/mprotect: use mmu_gather
Message-ID: <20211003121019.GF4323@worktop.programming.kicks-ass.net>
References: <20210925205423.168858-1-namit@vmware.com>
 <20210925205423.168858-2-namit@vmware.com>
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On Sat, Sep 25, 2021 at 01:54:22PM -0700, Nadav Amit wrote:

> @@ -338,25 +344,25 @@ static unsigned long change_protection_range(struct vm_area_struct *vma,
>  	struct mm_struct *mm = vma->vm_mm;
>  	pgd_t *pgd;
>  	unsigned long next;
> -	unsigned long start = addr;
>  	unsigned long pages = 0;
> +	struct mmu_gather tlb;
>  
>  	BUG_ON(addr >= end);
>  	pgd = pgd_offset(mm, addr);
>  	flush_cache_range(vma, addr, end);
>  	inc_tlb_flush_pending(mm);

That seems unbalanced...

> +	tlb_gather_mmu(&tlb, mm);
> +	tlb_start_vma(&tlb, vma);
>  	do {
>  		next = pgd_addr_end(addr, end);
>  		if (pgd_none_or_clear_bad(pgd))
>  			continue;
> -		pages += change_p4d_range(vma, pgd, addr, next, newprot,
> +		pages += change_p4d_range(&tlb, vma, pgd, addr, next, newprot,
>  					  cp_flags);
>  	} while (pgd++, addr = next, addr != end);
>  
> -	/* Only flush the TLB if we actually modified any entries: */
> -	if (pages)
> -		flush_tlb_range(vma, start, end);
> -	dec_tlb_flush_pending(mm);

... seeing you do remove the extra decrement.

> +	tlb_end_vma(&tlb, vma);
> +	tlb_finish_mmu(&tlb);
>  
>  	return pages;
>  }
> -- 
> 2.25.1
>