Re: [PATCH v27 3/4] mm/page_poison: expose page_poisoning_enabled to kernel modules

[Wei Wang]

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@xxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> > Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > ---
> >   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