linux-kernel - Re: [PATCH] mm: fixup validation of buddy pfn

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Date:   Tue, 21 Jun 2022 18:33:35 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Xianting Tian <xianting.tian@...ux.alibaba.com>
Cc:     David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
        guoren@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: fixup validation of buddy pfn

On 21 Jun 2022, at 4:49, Xianting Tian wrote:

> 在 2022/6/21 下午4:01, David Hildenbrand 写道:
>> On 21.06.22 05:11, Xianting Tian wrote:
>>> For RISC-V arch the first 2MB RAM could be reserved for opensbi,
>>> and the arch code may don't create pages for the first 2MB RAM,
>>> so it would have pfn_base=512 and mem_map began with 512th PFN when
>>> CONFIG_FLATMEM=y.
>>>
>>> But __find_buddy_pfn algorithm thinks the start PFN 0, it could get
>>> 0 PFN or less than the pfn_base value, so page_is_buddy() can't
>>> verify the page whose PFN is 0 ~ 511, actually we don't have valid
>>> pages for PFN 0 ~ 511.
>>>
>>> Actually, buddy system should not assume Arch cretaed pages for
>>> reserved memory, Arch may don't know the implied limitation.
>> Ehm, sorry, no. Archs have to stick to the rules of the buddy, not the
>> other way around. Why should we add additional overhead to the buddy
>> just because arch XYZ wants to be special?
>
> We ever sent a patch to create mapping for the first 2MB RAM for RISC-V, But it is not accetped.
>
> But I am just wondering, if we have the RAM whose physical base address is not 0, for example, start with 0x200000(2Mb).
>
> Then the base PFN is (0x200000 >> 12) = 512, Do we still need to create mapping for the non-existing first 2Mb RAM,
>
> if not, the issue still exist under the case?
>

How does RISC-V get mem_map for 2MB-4MB in FLATMEM? alloc_node_mem_map() from
mm/page_alloc.c only allocate MAX_ORDER_NR_PAGES aligned struct page.

>>
>> If at all, we should fail hard if an arch doesn't play with the rules
>> and make this a VM_BUG_ON().
>>
>>> With this patch, we can gurantee a valid buddy no matter what we
>>> have pages for reserved memory or not.
>>>
>>> Fixes: 8170ac4700d26f65 ("mm: wrap __find_buddy_pfn() with a necessary buddy page validation")
>>> Signed-off-by: Xianting Tian <xianting.tian@...ux.alibaba.com>
>>> ---
>>>   mm/internal.h | 6 +++++-
>>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/internal.h b/mm/internal.h
>>> index c0f8fbe0445b..0ec446caeb2e 100644
>>> --- a/mm/internal.h
>>> +++ b/mm/internal.h
>>> @@ -322,7 +322,8 @@ __find_buddy_pfn(unsigned long page_pfn, unsigned int order)
>>>    * The found buddy can be a non PageBuddy, out of @page's zone, or its order is
>>>    * not the same as @page. The validation is necessary before use it.
>>>    *
>>> - * Return: the found buddy page or NULL if not found.
>>> + * Return: the found buddy page or NULL if not found or NULL if buddy pfn is
>>> + *         not valid.
>>>    */
>>>   static inline struct page *find_buddy_page_pfn(struct page *page,
>>>   			unsigned long pfn, unsigned int order, unsigned long *buddy_pfn)
>>> @@ -330,6 +331,9 @@ static inline struct page *find_buddy_page_pfn(struct page *page,
>>>   	unsigned long __buddy_pfn = __find_buddy_pfn(pfn, order);
>>>   	struct page *buddy;
>>>  +	if (!pfn_valid(__buddy_pfn))
>>> +		return NULL;
>>> +
>>>   	buddy = page + (__buddy_pfn - pfn);
>>>   	if (buddy_pfn)
>>>   		*buddy_pfn = __buddy_pfn;
>>


--
Best Regards,
Yan, Zi

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