Re: [PATCH v2 4/6] mm/mremap: Use mmu gather interface instead of flush_tlb_range

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Aneesh Kumar K.V's message of March 15, 2021 9:38 pm:
> Some architectures do have the concept of page walk cache and only mmu gather
> interface supports flushing them. A fast mremap that involves moving page
> table pages instead of copying pte entries should flush page walk cache since
> the old translation cache is no more valid. Hence switch to mm gather to flush
> TLB and mark tlb.freed_tables = 1. No page table pages need to be freed here.
> With this the tlb flush is done outside page table lock (ptl).

I would maybe just get archs that implement it to provide a specific
flush_tlb+pwc_range for it, or else they get flush_tlb_range by default.

I think that would be simpler for now, at least in generic code.

There was some other talk of consolidating the TLB flush APIs, I jsut 
don't know if it's the best way to go to use the page/page table 
gathering and freeing API for it.

Thanks,
Nick

> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> ---
>  mm/mremap.c | 33 +++++++++++++++++++++++++++++----
>  1 file changed, 29 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 574287f9bb39..fafa73b965d3 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -216,6 +216,7 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
>  {
>  	spinlock_t *old_ptl, *new_ptl;
>  	struct mm_struct *mm = vma->vm_mm;
> +	struct mmu_gather tlb;
>  	pmd_t pmd;
>  
>  	/*
> @@ -244,11 +245,12 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
>  	if (WARN_ON_ONCE(!pmd_none(*new_pmd)))
>  		return false;
>  
> +	tlb_gather_mmu(&tlb, mm);
>  	/*
>  	 * We don't have to worry about the ordering of src and dst
>  	 * ptlocks because exclusive mmap_lock prevents deadlock.
>  	 */
> -	old_ptl = pmd_lock(vma->vm_mm, old_pmd);
> +	old_ptl = pmd_lock(mm, old_pmd);
>  	new_ptl = pmd_lockptr(mm, new_pmd);
>  	if (new_ptl != old_ptl)
>  		spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
> @@ -257,13 +259,23 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
>  	pmd = *old_pmd;
>  	pmd_clear(old_pmd);
>  
> +	/*
> +	 * Mark the range. We are not freeing page table pages nor
> +	 * regular pages. Hence we don't need to call tlb_remove_table()
> +	 * or tlb_remove_page().
> +	 */
> +	tlb_flush_pte_range(&tlb, old_addr, PMD_SIZE);
> +	tlb.freed_tables = 1;
>  	VM_BUG_ON(!pmd_none(*new_pmd));
>  	pmd_populate(mm, new_pmd, (pgtable_t)pmd_page_vaddr(pmd));
>  
> -	flush_tlb_range(vma, old_addr, old_addr + PMD_SIZE);
>  	if (new_ptl != old_ptl)
>  		spin_unlock(new_ptl);
>  	spin_unlock(old_ptl);
> +	/*
> +	 * This will invalidate both the old TLB and page table walk caches.
> +	 */
> +	tlb_finish_mmu(&tlb);
>  
>  	return true;
>  }
> @@ -282,6 +294,7 @@ static bool move_normal_pud(struct vm_area_struct *vma, unsigned long old_addr,
>  {
>  	spinlock_t *old_ptl, *new_ptl;
>  	struct mm_struct *mm = vma->vm_mm;
> +	struct mmu_gather tlb;
>  	pud_t pud;
>  
>  	/*
> @@ -291,11 +304,12 @@ static bool move_normal_pud(struct vm_area_struct *vma, unsigned long old_addr,
>  	if (WARN_ON_ONCE(!pud_none(*new_pud)))
>  		return false;
>  
> +	tlb_gather_mmu(&tlb, mm);
>  	/*
>  	 * We don't have to worry about the ordering of src and dst
>  	 * ptlocks because exclusive mmap_lock prevents deadlock.
>  	 */
> -	old_ptl = pud_lock(vma->vm_mm, old_pud);
> +	old_ptl = pud_lock(mm, old_pud);
>  	new_ptl = pud_lockptr(mm, new_pud);
>  	if (new_ptl != old_ptl)
>  		spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
> @@ -304,14 +318,25 @@ static bool move_normal_pud(struct vm_area_struct *vma, unsigned long old_addr,
>  	pud = *old_pud;
>  	pud_clear(old_pud);
>  
> +	/*
> +	 * Mark the range. We are not freeing page table pages nor
> +	 * regular pages. Hence we don't need to call tlb_remove_table()
> +	 * or tlb_remove_page().
> +	 */
> +	tlb_flush_pte_range(&tlb, old_addr, PUD_SIZE);
> +	tlb.freed_tables = 1;
>  	VM_BUG_ON(!pud_none(*new_pud));
>  
>  	pud_populate(mm, new_pud, (pmd_t *)pud_page_vaddr(pud));
> -	flush_tlb_range(vma, old_addr, old_addr + PUD_SIZE);
> +
>  	if (new_ptl != old_ptl)
>  		spin_unlock(new_ptl);
>  	spin_unlock(old_ptl);
>  
> +	/*
> +	 * This will invalidate both the old TLB and page table walk caches.
> +	 */
> +	tlb_finish_mmu(&tlb);
>  	return true;
>  }
>  #else
> -- 
> 2.29.2
> 
>