Re: [PATCH 1/2] mm/mprotect: use mmu_gather

Nadav Amit
2021-10-05T16:34:12Z

> On Oct 4, 2021, at 11:53 PM, Peter Zijlstra <peterz@infradead.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@infradead.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