Re: [PATCH v2] mm: use mem_map_offset instead of mem_map_next

[Mike Kravetz]

On 09/05/22 06:09, Cheng Li wrote:
> To handle discontiguity case, mem_map_next() has a parameter named
> `offset`. As a function caller, one would be confused why "get
> next entry" needs a parameter named "offset". The other drawback of
> mem_map_next() is that the callers must take care of the map between
> parameter "iter" and "offset", otherwise we may get an hole or
> duplication during iteration. So we use mem_map_offset instead of
> mem_map_next.
> 
> Signed-off-by: Cheng Li <lic121@xxxxxxxxxxxxxxx>
> Fixes: 69d177c2fc70 ("hugetlbfs: handle pages higher order than MAX_ORDER")

The Fixes tag implies there is a user visible bug.  I do not believe this is
the case here.  Is there a user visible bug?
-- 
Mike Kravetz

> ---
> 
> Notes:
>     v2:
>       - fix build error
> 
>  mm/hugetlb.c  | 25 +++++++++++++++----------
>  mm/internal.h | 16 ++--------------
>  mm/memory.c   | 21 ++++++++++-----------
>  3 files changed, 27 insertions(+), 35 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e070b8593b37..a9592f69bf82 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1308,12 +1308,13 @@ static void __destroy_compound_gigantic_page(struct page *page,
>  {
>  	int i;
>  	int nr_pages = 1 << order;
> -	struct page *p = page + 1;
> +	struct page *p;
>  
>  	atomic_set(compound_mapcount_ptr(page), 0);
>  	atomic_set(compound_pincount_ptr(page), 0);
>  
> -	for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
> +	for (i = 1; i < nr_pages; i++) {
> +		p = mem_map_offset(page, i);
>  		p->mapping = NULL;
>  		clear_compound_head(p);
>  		if (!demote)
> @@ -1530,7 +1531,7 @@ static void add_hugetlb_page(struct hstate *h, struct page *page,
>  static void __update_and_free_page(struct hstate *h, struct page *page)
>  {
>  	int i;
> -	struct page *subpage = page;
> +	struct page *subpage;
>  
>  	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
>  		return;
> @@ -1561,8 +1562,8 @@ static void __update_and_free_page(struct hstate *h, struct page *page)
>  	if (unlikely(PageHWPoison(page)))
>  		hugetlb_clear_page_hwpoison(page);
>  
> -	for (i = 0; i < pages_per_huge_page(h);
> -	     i++, subpage = mem_map_next(subpage, page, i)) {
> +	for (i = 0; i < pages_per_huge_page(h); i++) {
> +		subpage = mem_map_offset(page, i);
>  		subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
>  				1 << PG_referenced | 1 << PG_dirty |
>  				1 << PG_active | 1 << PG_private |
> @@ -1769,13 +1770,15 @@ static bool __prep_compound_gigantic_page(struct page *page, unsigned int order,
>  {
>  	int i, j;
>  	int nr_pages = 1 << order;
> -	struct page *p = page + 1;
> +	struct page *p;
>  
>  	/* we rely on prep_new_huge_page to set the destructor */
>  	set_compound_order(page, order);
>  	__ClearPageReserved(page);
>  	__SetPageHead(page);
> -	for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
> +	for (i = 1; i < nr_pages; i++) {
> +		p = mem_map_offset(page, i);
> +
>  		/*
>  		 * For gigantic hugepages allocated through bootmem at
>  		 * boot, it's safer to be consistent with the not-gigantic
> @@ -1822,14 +1825,16 @@ static bool __prep_compound_gigantic_page(struct page *page, unsigned int order,
>  
>  out_error:
>  	/* undo tail page modifications made above */
> -	p = page + 1;
> -	for (j = 1; j < i; j++, p = mem_map_next(p, page, j)) {
> +	for (j = 1; j < i; j++) {
> +		p = mem_map_offset(page, j);
>  		clear_compound_head(p);
>  		set_page_refcounted(p);
>  	}
>  	/* need to clear PG_reserved on remaining tail pages  */
> -	for (; j < nr_pages; j++, p = mem_map_next(p, page, j))
> +	for (; j < nr_pages; j++) {
> +		p = mem_map_offset(page, j);
>  		__ClearPageReserved(p);
> +	}
>  	set_compound_order(page, 0);
>  #ifdef CONFIG_64BIT
>  	page[1].compound_nr = 0;
> diff --git a/mm/internal.h b/mm/internal.h
> index 785409805ed7..1012a305a60f 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -646,25 +646,13 @@ static inline void vunmap_range_noflush(unsigned long start, unsigned long end)
>   */
>  static inline struct page *mem_map_offset(struct page *base, int offset)
>  {
> -	if (unlikely(offset >= MAX_ORDER_NR_PAGES))
> -		return nth_page(base, offset);
> -	return base + offset;
> -}
> -
> -/*
> - * Iterator over all subpages within the maximally aligned gigantic
> - * page 'base'.  Handle any discontiguity in the mem_map.
> - */
> -static inline struct page *mem_map_next(struct page *iter,
> -						struct page *base, int offset)
> -{
> -	if (unlikely((offset & (MAX_ORDER_NR_PAGES - 1)) == 0)) {
> +	if (unlikely(offset >= MAX_ORDER_NR_PAGES)) {
>  		unsigned long pfn = page_to_pfn(base) + offset;
>  		if (!pfn_valid(pfn))
>  			return NULL;
>  		return pfn_to_page(pfn);
>  	}
> -	return iter + 1;
> +	return base + offset;
>  }
>  
>  /* Memory initialisation debug and verification */
> diff --git a/mm/memory.c b/mm/memory.c
> index 4ba73f5aa8bb..32179c4fd1a5 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5637,11 +5637,11 @@ static void clear_gigantic_page(struct page *page,
>  				unsigned int pages_per_huge_page)
>  {
>  	int i;
> -	struct page *p = page;
> +	struct page *p;
>  
>  	might_sleep();
> -	for (i = 0; i < pages_per_huge_page;
> -	     i++, p = mem_map_next(p, page, i)) {
> +	for (i = 0; i < pages_per_huge_page; i++) {
> +		p = mem_map_offset(page, i);
>  		cond_resched();
>  		clear_user_highpage(p, addr + i * PAGE_SIZE);
>  	}
> @@ -5677,13 +5677,12 @@ static void copy_user_gigantic_page(struct page *dst, struct page *src,
>  	struct page *dst_base = dst;
>  	struct page *src_base = src;
>  
> -	for (i = 0; i < pages_per_huge_page; ) {
> +	for (i = 0; i < pages_per_huge_page; i++) {
> +		dst = mem_map_offset(dst_base, i);
> +		src = mem_map_offset(src_base, i);
> +
>  		cond_resched();
>  		copy_user_highpage(dst, src, addr + i*PAGE_SIZE, vma);
> -
> -		i++;
> -		dst = mem_map_next(dst, dst_base, i);
> -		src = mem_map_next(src, src_base, i);
>  	}
>  }
>  
> @@ -5730,10 +5729,10 @@ long copy_huge_page_from_user(struct page *dst_page,
>  	void *page_kaddr;
>  	unsigned long i, rc = 0;
>  	unsigned long ret_val = pages_per_huge_page * PAGE_SIZE;
> -	struct page *subpage = dst_page;
> +	struct page *subpage;
>  
> -	for (i = 0; i < pages_per_huge_page;
> -	     i++, subpage = mem_map_next(subpage, dst_page, i)) {
> +	for (i = 0; i < pages_per_huge_page; i++) {
> +		subpage = mem_map_offset(dst_page, i);
>  		if (allow_pagefault)
>  			page_kaddr = kmap(subpage);
>  		else
> -- 
> 1.8.3.1
>