linux-kernel - Re: [PATCH v27 3/4] mm/page_poison: expose page_poisoning

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Date:   Thu, 8 Feb 2018 05:04:43 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, mhocko@...nel.org, akpm@...ux-foundation.org,
        pbonzini@...hat.com, liliang.opensource@...il.com,
        yang.zhang.wz@...il.com, quan.xu0@...il.com, nilal@...hat.com,
        riel@...hat.com, huangzhichao@...wei.com
Subject: Re: [PATCH v27 3/4] mm/page_poison: expose page_poisoning_enabled to
 kernel modules

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@...el.com>
> > > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > > Cc: Michal Hocko <mhocko@...nel.org>
> > > Cc: Michael S. Tsirkin <mst@...hat.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