Re: [PATCHv2 2/2] mm: downgrade VM_BUG in isolate_lru_page() to warning

[Andrew Morton]

On Tue,  2 Feb 2016 19:21:01 +0300 "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:

> Calling isolate_lru_page() is wrong and shouldn't happen, but it not
> nessesary fatal: the page just will not be isolated if it's not on LRU.
> 
> Let's downgrade the VM_BUG_ON_PAGE() to WARN_RATELIMIT().
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index eb3dd37ccd7c..71b1c29948db 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
>  	int ret = -EBUSY;
>  
>  	VM_BUG_ON_PAGE(!page_count(page), page);
> -	VM_BUG_ON_PAGE(PageTail(page), page);
> +	WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");
>  
>  	if (PageLRU(page)) {
>  		struct zone *zone = page_zone(page);

Confused.  I thought mm-fix-bogus-vm_bug_on_page-in-isolate_lru_page.patch:

--- a/mm/vmscan.c~mm-fix-bogus-vm_bug_on_page-in-isolate_lru_page
+++ a/mm/vmscan.c
@@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
 	int ret = -EBUSY;
 
 	VM_BUG_ON_PAGE(!page_count(page), page);
-	VM_BUG_ON_PAGE(PageTail(page), page);
+	VM_BUG_ON_PAGE(PageLRU(page) && PageTail(page), page);
 
 	if (PageLRU(page)) {
 		struct zone *zone = page_zone(page);

was better.  We *know* that we sometimes encounter LRU pages here and
we know that we handle them correctly.  So why scare users by blurting
out a warning about something for which we won't be taking any action?