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

Peter Xu
2021-10-12T10:16:40Z

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