Re: [v4 PATCH 1/2] mm: thp: replace DEBUG_VM BUG with VM_WARN when unmap fails for split

[Hugh Dickins <hughd@google.com>]

On Wed, 26 May 2021, Yang Shi wrote:

> When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
> fail, but the first VM_BUG_ON_PAGE() just checks page_mapcount() however
> it may miss the failure when head page is unmapped but other subpage is
> mapped.  Then the second DEBUG_VM BUG() that check total mapcount would
> catch it.  This may incur some confusion.  And this is not a fatal issue,
> so consolidate the two DEBUG_VM checks into one VM_WARN_ON_ONCE_PAGE().
> 
> [1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@e16-tech.com/
> 
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Yang Shi <shy828301@gmail.com>

Acked-by: Hugh Dickins <hughd@google.com>

Thanks: and 2/2 already has my Ack, correct.

> ---
> v4: Updated the subject and commit log per Hugh.
>     Reordered the patches per Hugh.
> v3: Incorporated the comments from Hugh. Keep Zi Yan's reviewed-by tag
>     since there is no fundamental change against v2.
> v2: Removed dead code and updated the comment of try_to_unmap() per Zi
>     Yan.
> 
>  mm/huge_memory.c | 24 +++++++-----------------
>  1 file changed, 7 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 19195fca1aee..8827f82c5302 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2336,15 +2336,15 @@ 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);
> +
> +	VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
>  }
>  
>  static void remap_page(struct page *page, unsigned int nr)
> @@ -2655,7 +2655,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  	struct deferred_split *ds_queue = get_deferred_split_queue(head);
>  	struct anon_vma *anon_vma = NULL;
>  	struct address_space *mapping = NULL;
> -	int count, mapcount, extra_pins, ret;
> +	int extra_pins, ret;
>  	pgoff_t end;
>  
>  	VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
> @@ -2714,7 +2714,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  	}
>  
>  	unmap_page(head);
> -	VM_BUG_ON_PAGE(compound_mapcount(head), head);
>  
>  	/* block interrupt reentry in xa_lock and spinlock */
>  	local_irq_disable();
> @@ -2732,9 +2731,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  
>  	/* Prevent deferred_split_scan() touching ->_refcount */
>  	spin_lock(&ds_queue->split_queue_lock);
> -	count = page_count(head);
> -	mapcount = total_mapcount(head);
> -	if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) {
> +	if (page_ref_freeze(head, 1 + extra_pins)) {
>  		if (!list_empty(page_deferred_list(head))) {
>  			ds_queue->split_queue_len--;
>  			list_del(page_deferred_list(head));
> @@ -2754,16 +2751,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  		__split_huge_page(page, list, end);
>  		ret = 0;
>  	} else {
> -		if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
> -			pr_alert("total_mapcount: %u, page_count(): %u\n",
> -					mapcount, count);
> -			if (PageTail(page))
> -				dump_page(head, NULL);
> -			dump_page(page, "total_mapcount(head) > 0");
> -			BUG();
> -		}
>  		spin_unlock(&ds_queue->split_queue_lock);
> -fail:		if (mapping)
> +fail:
> +		if (mapping)
>  			xa_unlock(&mapping->i_pages);
>  		local_irq_enable();
>  		remap_page(head, thp_nr_pages(head));
> -- 
> 2.26.2
> 
>