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

[Nadav Amit <nadav.amit@gmail.com>]

> On Oct 12, 2021, at 3:16 AM, Peter Xu <peterx@redhat.com> 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);
>> +	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?

Good point.

tlb_start_vma() and tlb_end_vma() are required since some archs do not
batch TLB flushes across VMAs (e.g., ARM). I am not sure whether that’s
the best behavior for all archs, but I do not want to change it.

Anyhow, you make a valid point that the flush_cache_range() should be
dropped as well. I will do so for next version.

Regards,
Nadav