Re: [RFC 2/2] page-flags: Catch the double setter of page flags

[Linux Upstream <linux.upstream@oneplus.com>]

On 11/02/19 7:17 PM, Peter Zijlstra wrote:
> On Mon, Feb 11, 2019 at 12:53:55PM +0000, Chintan Pandya wrote:
>> Some of the page flags, like PG_locked is not supposed to
>> be set twice. Currently, there is no protection around this
>> and many callers directly tries to set this bit. Others
>> follow trylock_page() which is much safer version of the
>> same. But, for performance issues, we may not want to
>> implement wait-until-set. So, at least, find out who is
>> doing double setting and fix them.
>>
>> Change-Id: I1295fcb8527ce4b54d5d11c11287fc7516006cf0
>> Signed-off-by: Chintan Pandya <chintan.pandya@oneplus.com>
>> ---
>>   include/linux/page-flags.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index a56a9bd4bc6b..e307775c2b4a 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
>> @@ -208,7 +208,7 @@ static __always_inline int Page##uname(struct page *page)		\
>>   
>>   #define SETPAGEFLAG(uname, lname, policy)				\
>>   static __always_inline void SetPage##uname(struct page *page)		\
>> -	{ set_bit(PG_##lname, &policy(page, 1)->flags); }
>> +	{ WARN_ON(test_and_set_bit(PG_##lname, &policy(page, 1)->flags)); }
> 
> You forgot to make this depend on CONFIG_DEBUG_VM. Also, I'm not
> convinced this is always wrong, inefficient sure, but not wrong in
> general.

Okay. Will protect this under CONFIG_DEBUG_VM.