linux-kernel - Re: [PATCH 1/2] mm/mprotect: use mmu

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Message-ID: <YWVgdmKIFnZcgjeY@t490s>
Date:   Tue, 12 Oct 2021 18:16:22 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, Nadav Amit <namit@...are.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Nick Piggin <npiggin@...il.com>, x86@...nel.org
Subject: Re: [PATCH 1/2] mm/mprotect: use mmu_gather

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);
> +	tlb_gather_mmu(&tlb, mm);
> +	tlb_start_vma(&tlb, vma);

Pure question:

I actually have no idea why tlb_start_vma() is needed here, as protection range
can be just a single page, but anyway.. I do see that tlb_start_vma() contains
a whole-vma flush_cache_range() when the arch needs it, then does it mean that
besides the inc_tlb_flush_pending() to be dropped, so as to the other call to
flush_cache_range() above?

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

-- 
Peter Xu