Re: [PATCH] mm: fixup validation of buddy pfn

[Xianting Tian]

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?

> 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@xxxxxxxxxxxxxxxxx>
> > ---
> >   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;