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

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Date:   Thu, 08 Feb 2018 09:44:29 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.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 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?

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