Linux Virtualization - Feb 2018 - [PATCH v27 3/4] mm/page_poison: expose page_poisoning_enabled to kernel modules

On Wed, Feb 07, 2018 at 02:54:30PM +0800, Wei Wang
wrote:
> In some usages, e.g. virtio-balloon, a kernel module needs to know if
> page poisoning is in use. This patch exposes the page_poisoning_enabled
> function to kernel modules.
> 
> Signed-off-by: Wei Wang <wei.w.wang at intel.com>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> Cc: Michal Hocko <mhocko at kernel.org>
> Cc: Michael S. Tsirkin <mst at redhat.com>
> ---
>  mm/page_poison.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/page_poison.c b/mm/page_poison.c
> index e83fd44..c08d02a 100644
> --- a/mm/page_poison.c
> +++ b/mm/page_poison.c
> @@ -30,6 +30,11 @@ bool page_poisoning_enabled(void)
>  		debug_pagealloc_enabled()));
>  }
>  
> +/**
> + * page_poisoning_enabled - check if page poisoning is enabled
> + *
> + * Return true if page poisoning is enabled, or false if not.
> + */
>  static void poison_page(struct page *page)
>  {
>  	void *addr = kmap_atomic(page);
> @@ -37,6 +42,7 @@ static void poison_page(struct page *page)
>  	memset(addr, PAGE_POISON, PAGE_SIZE);
>  	kunmap_atomic(addr);
>  }
> +EXPORT_SYMBOL_GPL(page_poisoning_enabled);
>  
>  static void poison_pages(struct page *page, int n)
>  {
Looks like both the comment and the export are in the wrong place.
I'm a bit concerned that callers also in fact poke at the
PAGE_POISON - exporting that seems to be more of an accident
as it's only used without page_poisoning.c - it might be
better to have page_poisoning_enabled get u8 * and set it.
> -- 
> 2.7.4

On 02/08/2018 02:34 AM, Michael S. Tsirkin wrote:
> On Wed, Feb 07, 2018 at 02:54:30PM +0800, Wei Wang wrote:
>> In some usages, e.g. virtio-balloon, a kernel module needs to know if
>> page poisoning is in use. This patch exposes the page_poisoning_enabled
>> function to kernel modules.
>>
>> Signed-off-by: Wei Wang <wei.w.wang at intel.com>
>> Cc: Andrew Morton <akpm at linux-foundation.org>
>> Cc: Michal Hocko <mhocko at kernel.org>
>> Cc: Michael S. Tsirkin <mst at redhat.com>
>> ---
>>   mm/page_poison.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/mm/page_poison.c b/mm/page_poison.c
>> index e83fd44..c08d02a 100644
>> --- a/mm/page_poison.c
>> +++ b/mm/page_poison.c
>> @@ -30,6 +30,11 @@ bool page_poisoning_enabled(void)
>>   		debug_pagealloc_enabled()));
>>   }
>>   
>> +/**
>> + * page_poisoning_enabled - check if page poisoning is enabled
>> + *
>> + * Return true if page poisoning is enabled, or false if not.
>> + */
>>   static void poison_page(struct page *page)
>>   {
>>   	void *addr = kmap_atomic(page);
>> @@ -37,6 +42,7 @@ static void poison_page(struct page *page)
>>   	memset(addr, PAGE_POISON, PAGE_SIZE);
>>   	kunmap_atomic(addr);
>>   }
>> +EXPORT_SYMBOL_GPL(page_poisoning_enabled);
>>   
>>   static void poison_pages(struct page *page, int n)
>>   {
> Looks like both the comment and the export are in the wrong place.
Thanks. Will be more careful.
> I'm a bit concerned that callers also in fact poke at the
> PAGE_POISON - exporting that seems to be more of an accident
> as it's only used without page_poisoning.c - it might be
> better to have page_poisoning_enabled get u8 * and set it.
>
PAGE_POISON is a macro defined in the header, why would callers using it 
be a concern?

Do you suggest to have:

bool page_poisoning_get(u8 *val)
{
     if (page_poisoning_enabled()) {
         *val = PAGE_POISON;
         return true;
     }

     return false;
}
EXPORT_SYMBOL_GPL(page_poisoning_get);


Best,
Wei

On Thu, Feb 08, 2018 at 09:44:29AM +0800, Wei Wang
wrote:
> On 02/08/2018 02:34 AM, Michael S. Tsirkin wrote:
> > On Wed, Feb 07, 2018 at 02:54:30PM +0800, Wei Wang wrote:
> > > In some usages, e.g. virtio-balloon, a kernel module needs to
know if
> > > page poisoning is in use. This patch exposes the
page_poisoning_enabled
> > > function to kernel modules.
> > > 
> > > Signed-off-by: Wei Wang <wei.w.wang at intel.com>
> > > Cc: Andrew Morton <akpm at linux-foundation.org>
> > > Cc: Michal Hocko <mhocko at kernel.org>
> > > Cc: Michael S. Tsirkin <mst at redhat.com>
> > > ---
> > >   mm/page_poison.c | 6 ++++++
> > >   1 file changed, 6 insertions(+)
> > > 
> > > diff --git a/mm/page_poison.c b/mm/page_poison.c
> > > index e83fd44..c08d02a 100644
> > > --- a/mm/page_poison.c
> > > +++ b/mm/page_poison.c
> > > @@ -30,6 +30,11 @@ bool page_poisoning_enabled(void)
> > >   		debug_pagealloc_enabled()));
> > >   }
> > > +/**
> > > + * page_poisoning_enabled - check if page poisoning is enabled
> > > + *
> > > + * Return true if page poisoning is enabled, or false if not.
> > > + */
> > >   static void poison_page(struct page *page)
> > >   {
> > >   	void *addr = kmap_atomic(page);
> > > @@ -37,6 +42,7 @@ static void poison_page(struct page *page)
> > >   	memset(addr, PAGE_POISON, PAGE_SIZE);
> > >   	kunmap_atomic(addr);
> > >   }
> > > +EXPORT_SYMBOL_GPL(page_poisoning_enabled);
> > >   static void poison_pages(struct page *page, int n)
> > >   {
> > Looks like both the comment and the export are in the wrong place.
> 
> Thanks. Will be more careful.
> 
> > I'm a bit concerned that callers also in fact poke at the
> > PAGE_POISON - exporting that seems to be more of an accident
> > as it's only used without page_poisoning.c - it might be
> > better to have page_poisoning_enabled get u8 * and set it.
> > 
> 
> PAGE_POISON is a macro defined in the header, why would callers using it be
> a concern?

It might be a good idea to move it out of there though.
> Do you suggest to have:
> 
> bool page_poisoning_get(u8 *val)
> {
>     if (page_poisoning_enabled()) {
>         *val = PAGE_POISON;
>         return true;
>     }
> 
>     return false;
> }
> EXPORT_SYMBOL_GPL(page_poisoning_get);
> 
> 
> Best,
> Wei
Something like this, yes.

-- 
MST