[PATCH] mm: rectify a page bad reason

[PATCH] mm: rectify a page bad reason

[Rongwei Wang]

Hi

When I was doing some memory-related projects, it always reported error
"nonzero mapcount", but its judgment condition was that _mapcount was not equal
to -1, so I felt the original string was a bit inappropriate, so I tried to
update it.

Signed-off-by: Rongwei Wang <rongwei.wang@linux.alibaba.com>
---
 mm/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7a2c89b..57d7f26 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1114,7 +1114,7 @@ static const char *page_bad_reason(struct page *page, unsigned long flags)
 	const char *bad_reason = NULL;
 
 	if (unlikely(atomic_read(&page->_mapcount) != -1))
-		bad_reason = "nonzero mapcount";
+		bad_reason = "non-(-1) _mapcount";
 	if (unlikely(page->mapping != NULL))
 		bad_reason = "non-NULL mapping";
 	if (unlikely(page_ref_count(page) != 0))
-- 
1.8.3.1

Re: [PATCH] mm: rectify a page bad reason

[Rongwei Wang]

> On Dec 29, 2020, at 11:21 AM, Muchun Song <smuchun@gmail.com> wrote:
> 
> Rongwei Wang <rongwei.wang@linux.alibaba.com> 于2020年12月29日周二 上午11:16写道：
>> 
>> Hi
>> 
>> When I was doing some memory-related projects, it always reported error
>> "nonzero mapcount", but its judgment condition was that _mapcount was not equal
>> to -1, so I felt the original string was a bit inappropriate, so I tried to
>> update it.
> 
> Hi Rongwei,
> 
> Because the page_mapcount() just returns atomic_read(&page->_mapcount) + 1,
> reporting "nonzero mapcount" is reasonable when _mapcount is -1.
Hi, Muchun

Thank you for the tip. I read the __dump_page function again.
Indeed here need to combine page_mapcount+page_bad_reason to understand, and reporting "nonzero mapcount" is reasonable. It always feel a little strange ONLY look at page_bad_reason.

Maybe it should be changed (or NOT) to this:

if (unlikely(page_mapcount(page) != 0))
	bad_reason = "nonzero mapcount”;

Anyway, thanks
Rongwei Wang
> 
> Thanks.
> 
>> 
>> Signed-off-by: Rongwei Wang <rongwei.wang@linux.alibaba.com>
>> ---
>> mm/page_alloc.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 7a2c89b..57d7f26 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1114,7 +1114,7 @@ static const char *page_bad_reason(struct page *page, unsigned long flags)
>>        const char *bad_reason = NULL;
>> 
>>        if (unlikely(atomic_read(&page->_mapcount) != -1))
>> -               bad_reason = "nonzero mapcount";
>> +               bad_reason = "non-(-1) _mapcount";
>>        if (unlikely(page->mapping != NULL))
>>                bad_reason = "non-NULL mapping";
>>        if (unlikely(page_ref_count(page) != 0))
>> --
>> 1.8.3.1

Re: [PATCH] mm: rectify a page bad reason

[Matthew Wilcox]

On Tue, Dec 29, 2020 at 11:11:56AM +0800, Rongwei Wang wrote:
> When I was doing some memory-related projects, it always reported error
> "nonzero mapcount", but its judgment condition was that _mapcount was not equal
> to -1, so I felt the original string was a bit inappropriate, so I tried to
> update it.

But '_mapcount' of -1 _is_ a mapcount of 0.  If we need to improve the
documentation somewhere, that'd be better than changing this message.

I do wonder if we want to add:

	if (unlikely(page_has_type(page))
		bad_reason = "page still typed";

It's covered by the non-zero mapcount case, but is slightly misleading.

>  	if (unlikely(atomic_read(&page->_mapcount) != -1))
> -		bad_reason = "nonzero mapcount";
> +		bad_reason = "non-(-1) _mapcount";
>  	if (unlikely(page->mapping != NULL))
>  		bad_reason = "non-NULL mapping";
>  	if (unlikely(page_ref_count(page) != 0))
> -- 
> 1.8.3.1
> 
>