Re: [v3 PATCH 1/2] mm: rmap: make try_to_unmap() void function

[Minchan Kim <minchan@kernel.org>]

On Tue, May 25, 2021 at 09:21:44AM -0700, Yang Shi wrote:
> Currently try_to_unmap() return bool value by checking page_mapcount(),
> however this may return false positive since page_mapcount() doesn't
> check all subpages of compound page.  The total_mapcount() could be used
> instead, but its cost is higher since it traverses all subpages.
> 
> Actually the most callers of try_to_unmap() don't care about the
> return value at all.  So just need check if page is still mapped by
> page_mapped() when necessary.  And page_mapped() does bail out early
> when it finds mapped subpage.
> 
> Suggested-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
>  include/linux/rmap.h |  2 +-
>  mm/huge_memory.c     |  4 +---
>  mm/memory-failure.c  | 13 ++++++-------
>  mm/rmap.c            |  6 +-----
>  mm/vmscan.c          |  3 ++-
>  5 files changed, 11 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index def5c62c93b3..116cb193110a 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -194,7 +194,7 @@ static inline void page_dup_rmap(struct page *page, bool compound)
>  int page_referenced(struct page *, int is_locked,
>  			struct mem_cgroup *memcg, unsigned long *vm_flags);
>  
> -bool try_to_unmap(struct page *, enum ttu_flags flags);
> +void try_to_unmap(struct page *, enum ttu_flags flags);
>  
>  /* Avoid racy checks */
>  #define PVMW_SYNC		(1 << 0)
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 19195fca1aee..80fe642d742d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2336,15 +2336,13 @@ static void unmap_page(struct page *page)
>  {
>  	enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK |
>  		TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
> -	bool unmap_success;
>  
>  	VM_BUG_ON_PAGE(!PageHead(page), page);
>  
>  	if (PageAnon(page))
>  		ttu_flags |= TTU_SPLIT_FREEZE;
>  
> -	unmap_success = try_to_unmap(page, ttu_flags);
> -	VM_BUG_ON_PAGE(!unmap_success, page);
> +	try_to_unmap(page, ttu_flags);
>  }
>  
>  static void remap_page(struct page *page, unsigned int nr)
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 9dcc9bcea731..6dd53ff34825 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1126,7 +1126,7 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
>  		collect_procs(hpage, &tokill, flags & MF_ACTION_REQUIRED);
>  
>  	if (!PageHuge(hpage)) {
> -		unmap_success = try_to_unmap(hpage, ttu);
> +		try_to_unmap(hpage, ttu);
>  	} else {
>  		if (!PageAnon(hpage)) {
>  			/*
> @@ -1138,17 +1138,16 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
>  			 */
>  			mapping = hugetlb_page_mapping_lock_write(hpage);
>  			if (mapping) {
> -				unmap_success = try_to_unmap(hpage,
> -						     ttu|TTU_RMAP_LOCKED);
> +				try_to_unmap(hpage, ttu|TTU_RMAP_LOCKED);
>  				i_mmap_unlock_write(mapping);
> -			} else {
> +			} else
>  				pr_info("Memory failure: %#lx: could not lock mapping for mapped huge page\n", pfn);
> -				unmap_success = false;
> -			}
>  		} else {
> -			unmap_success = try_to_unmap(hpage, ttu);
> +			try_to_unmap(hpage, ttu);
>  		}
>  	}
> +
> +	unmap_success = !page_mapped(hpage);
>  	if (!unmap_success)
>  		pr_err("Memory failure: %#lx: failed to unmap page (mapcount=%d)\n",
>  		       pfn, page_mapcount(hpage));
> diff --git a/mm/rmap.c b/mm/rmap.c
> index a35cbbbded0d..728de421e43a 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1748,10 +1748,8 @@ static int page_not_mapped(struct page *page)
>   *
>   * Tries to remove all the page table entries which are mapping this
>   * page, used in the pageout path.  Caller must hold the page lock.
> - *
> - * If unmap is successful, return true. Otherwise, false.
>   */
> -bool try_to_unmap(struct page *page, enum ttu_flags flags)
> +void try_to_unmap(struct page *page, enum ttu_flags flags)
>  {
>  	struct rmap_walk_control rwc = {
>  		.rmap_one = try_to_unmap_one,
> @@ -1776,8 +1774,6 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
>  		rmap_walk_locked(page, &rwc);
>  	else
>  		rmap_walk(page, &rwc);
> -
> -	return !page_mapcount(page) ? true : false;

Couldn't we use page_mapped instead of page_mapcount here?
With boolean return of try sematic looks reasonable to me
rather than void.

>  }
>  
>  /**
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f96d62159720..fa5052ace415 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1499,7 +1499,8 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>  			if (unlikely(PageTransHuge(page)))
>  				flags |= TTU_SPLIT_HUGE_PMD;
>  
> -			if (!try_to_unmap(page, flags)) {
> +			try_to_unmap(page, flags);
> +			if (page_mapped(page)) {
>  				stat->nr_unmap_fail += nr_pages;
>  				if (!was_swapbacked && PageSwapBacked(page))
>  					stat->nr_lazyfree_fail += nr_pages;
> -- 
> 2.26.2
> 
>