linux-kernel - Re: [PATCH v2 2/2] mm: wrap __find_buddy_pfn() with a necessary buddy page validation.

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Date:   Sat, 2 Apr 2022 00:33:00 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Zi Yan <ziy@...dia.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        David Hildenbrand <david@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Rapoport <rppt@...nel.org>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] mm: wrap __find_buddy_pfn() with a necessary buddy
 page validation.

On 4/1/22 21:25, Zi Yan wrote:
> On 1 Apr 2022, at 15:01, Linus Torvalds wrote:
> 
>> On Fri, Apr 1, 2022 at 11:56 AM Zi Yan <ziy@...dia.com> wrote:
>>>
>>> How about the patch below? If it looks good, I will send v3.
>>
>> I can't see anything worrisome, but by now I've looked at several
>> versions and who knows what I'm missing.
>>
>> Making it inline and allowing a NULL 'buddy_pfn' pointer for the cases
>> that don't care might be an option, but I don't think it matters
>> hugely.
> 
> What do you mean by inlining it? Moving the function and __find_buddy_pfn()
> to mm/internal.h and mark both inline?

I would prefer that for the sake of __free_one_page().

> Something like below to allow a NULL 'buddy_pfn'? buddy_pfn is needed
> to store the result of __find_buddy_pfn(). The code does not look
> as nice as before.
> 
> struct page *find_buddy_page_pfn(struct page *page, unsigned long pfn,
>                         unsigned int order, unsigned long *buddy_pfn)
> {
>         struct page *buddy;
> 
>         if (buddy_pfn) {
>                 *buddy_pfn = __find_buddy_pfn(pfn, order);
>                 buddy = page + (*buddy_pfn - pfn);
>         } else
>                 buddy = page + (__find_buddy_pfn(pfn, order) - pfn);
> 
>         if (page_is_buddy(page, buddy, order))
>                 return buddy;
>         return NULL;
> }
> 
> or
> 
> struct page *find_buddy_page_pfn(struct page *page, unsigned long pfn,
>                         unsigned int order, unsigned long *buddy_pfn)
> {
>         struct page *buddy;
>         unsigned long local_buddy_pfn = __find_buddy_pfn(pfn, order);
> 
>         buddy = page + (local_buddy_pfn - pfn);
>         if (buddy_pfn)
>                 *buddy_pfn = local_buddy_pfn;
> 
>         if (page_is_buddy(page, buddy, order))
>                 return buddy;
>         return NULL;
> }

The latter looks better. I would also use a name e.g. '__buddy_pfn'
instead of 'local_'.

Thanks.

> 
> --
> Best Regards,
> Yan, Zi