Re: [PATCH 3/6] mm/mremap: Convert huge PUD move to separate helper

[Hugh Dickins <hughd@google.com>]

On Thu, 10 Jun 2021, Aneesh Kumar K.V wrote:

> With TRANSPARENT_HUGEPAGE_PUD enabled the kernel can find huge PUD entries.
> Add a helper to move huge PUD entries on mremap().
> 
> This will be used by a later patch to optimize mremap of PUD_SIZE aligned
> level 4 PTE mapped address
> 
> This also make sure we support mremap on huge PUD entries even with
> CONFIG_HAVE_MOVE_PUD disabled.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> ---
>  mm/mremap.c | 80 ++++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 73 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 47c255b60150..92ab7d24a587 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -324,10 +324,62 @@ static inline bool move_normal_pud(struct vm_area_struct *vma,
>  }
>  #endif
>  
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE_PUD

Should that say
#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
?

(I'm a PUD-THP-sceptic, but if it's just for DAX then probably okay.)

> +static bool move_huge_pud(struct vm_area_struct *vma, unsigned long old_addr,
> +			  unsigned long new_addr, pud_t *old_pud, pud_t *new_pud)
> +{
> +	spinlock_t *old_ptl, *new_ptl;
> +	struct mm_struct *mm = vma->vm_mm;
> +	pud_t pud;
> +
> +	/*
> +	 * The destination pud shouldn't be established, free_pgtables()
> +	 * should have released it.
> +	 */
> +	if (WARN_ON_ONCE(!pud_none(*new_pud)))
> +		return false;
> +
> +	/*
> +	 * 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);
> +	new_ptl = pud_lockptr(mm, new_pud);
> +	if (new_ptl != old_ptl)
> +		spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
> +
> +	/* Clear the pud */
> +	pud = *old_pud;
> +	pud_clear(old_pud);
> +
> +	VM_BUG_ON(!pud_none(*new_pud));
> +
> +	/* Set the new pud */
> +	/* mark soft_ditry when we add pud level soft dirty support */
> +	set_pud_at(mm, new_addr, new_pud, pud);
> +	flush_pud_tlb_range(vma, old_addr, old_addr + HPAGE_PUD_SIZE);
> +	if (new_ptl != old_ptl)
> +		spin_unlock(new_ptl);
> +	spin_unlock(old_ptl);
> +
> +	return true;
> +}
> +#else
> +static bool move_huge_pud(struct vm_area_struct *vma, unsigned long old_addr,
> +			  unsigned long new_addr, pud_t *old_pud, pud_t *new_pud)
> +{
> +	WARN_ON_ONCE(1);
> +	return false;
> +
> +}
> +#endif
> +
>  enum pgt_entry {
>  	NORMAL_PMD,
>  	HPAGE_PMD,
>  	NORMAL_PUD,
> +	HPAGE_PUD,
>  };
>  
>  /*
> @@ -347,6 +399,7 @@ static __always_inline unsigned long get_extent(enum pgt_entry entry,
>  		mask = PMD_MASK;
>  		size = PMD_SIZE;
>  		break;
> +	case HPAGE_PUD:
>  	case NORMAL_PUD:
>  		mask = PUD_MASK;
>  		size = PUD_SIZE;
> @@ -395,6 +448,11 @@ static bool move_pgt_entry(enum pgt_entry entry, struct vm_area_struct *vma,
>  			move_huge_pmd(vma, old_addr, new_addr, old_entry,
>  				      new_entry);
>  		break;
> +	case HPAGE_PUD:
> +		moved = move_huge_pud(vma, old_addr, new_addr, old_entry,
> +				      new_entry);
> +		break;
> +
>  	default:
>  		WARN_ON_ONCE(1);
>  		break;
> @@ -414,6 +472,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>  	unsigned long extent, old_end;
>  	struct mmu_notifier_range range;
>  	pmd_t *old_pmd, *new_pmd;
> +	pud_t *old_pud, *new_pud;
>  
>  	old_end = old_addr + len;
>  	flush_cache_range(vma, old_addr, old_end);
> @@ -429,15 +488,22 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>  		 * PUD level if possible.
>  		 */
>  		extent = get_extent(NORMAL_PUD, old_addr, old_end, new_addr);
> -		if (IS_ENABLED(CONFIG_HAVE_MOVE_PUD) && extent == PUD_SIZE) {
> -			pud_t *old_pud, *new_pud;
>  
> -			old_pud = get_old_pud(vma->vm_mm, old_addr);
> -			if (!old_pud)
> +		old_pud = get_old_pud(vma->vm_mm, old_addr);
> +		if (!old_pud)
> +			continue;
> +		new_pud = alloc_new_pud(vma->vm_mm, vma, new_addr);
> +		if (!new_pud)
> +			break;
> +		if (pud_trans_huge(*old_pud) || pud_devmap(*old_pud)) {
> +			if (extent == HPAGE_PUD_SIZE) {
> +				move_pgt_entry(HPAGE_PUD, vma, old_addr, new_addr,
> +					       old_pud, new_pud, need_rmap_locks);
> +				/* We ignore and continue on error? */
>  				continue;
> -			new_pud = alloc_new_pud(vma->vm_mm, vma, new_addr);
> -			if (!new_pud)
> -				break;
> +			}
> +		} else if (IS_ENABLED(CONFIG_HAVE_MOVE_PUD) && extent == PUD_SIZE) {
> +
>  			if (move_pgt_entry(NORMAL_PUD, vma, old_addr, new_addr,
>  					   old_pud, new_pud, need_rmap_locks))
>  				continue;
> -- 
> 2.31.1
> 
> 
>