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

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Date:   Tue, 5 Oct 2021 09:34:06 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        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 Oct 4, 2021, at 11:53 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Mon, Oct 04, 2021 at 12:24:14PM -0700, Nadav Amit wrote:
>> 
>> 
>>> On Oct 3, 2021, at 5:10 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>> 
>>> 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...
>> 
>> Bad rebase. Thanks for catching it!
>> 
>>> 
>>>> +	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.
>> 
>> Is it really needed? We do not put this comment elsewhere for
>> tlb_finish_mmu(). But no problem, I’ll keep it.
> 
> -ENOPARSE, did you read decrement as comment? In any case, I don't
> particularly care about the comment, and tlb_*_mmu() imply the inc/dec
> thingies.
> 
> All I tried to do is point out that removing the dec but leaving the inc
> is somewhat inconsistent :-)

The autocorrect in my mind was broken so I read as “documentation”
instead of “decrement”.

I will send v2 soon.

Thanks again!
Nadav