Re: [PATCH] mm: vmalloc: make vmalloc_to_page() deal with PMD/PUD mappings

[Dave Hansen <dave.hansen@intel.com>]

On 06/02/2017 09:21 AM, Ard Biesheuvel wrote:
>> First of all, this math isn't guaranteed to work.  We don't guarantee
>> virtual contiguity for all mem_map[]s.  I think you need to go to a pfn
>> or paddr first, add the pud offset, then convert to a 'struct page'.
> 
> OK, so you are saying the slice of the struct page array covering the
> range could be discontiguous even though the physical range it
> describes is contiguous? (which is guaranteed due to the nature of a
> PMD mapping IIUC) In that case,

Yes.

>> But, what *is* the right thing to return here?  Do the users here want
>> the head page or the tail page?
> 
> Hmm, I see what you mean. The vread() code that I am trying to fix
> simply kmaps the returned page, copies from it and unmaps it, so it is
> after the tail page. But I guess code that is aware of compound pages
> is after the head page instead.

Yeah, and some operations happen on tail pages while others get
redirected to the head page.

>> BTW, _are_ your huge vmalloc pages compound?
> 
> Not in the case that I am trying to solve, no. They are simply VM_MAP
> mappings of sequences of pages that are occupied by the kernel itself,
> and not allocated by the page allocator.

Huh, so what are they?  Are they system RAM that was bootmem allocated
or something?

>>>>> +#else
>>>>> +     VIRTUAL_BUG_ON(1);
>>>>> +#endif
>>>>> +     return page;
>>>>> +}
>>>> So if somebody manages to call this function on a huge page table entry,
>>>> but doesn't have hugetlbfs configured on, we kill the machine?
>>> Yes. But only if you have CONFIG_DEBUG_VIRTUAL defined, in which case
>>> it seems appropriate to signal a failure rather than proceed with
>>> dereferencing the huge PMD entry as if it were a table entry.
>>
>> Why kill the machine rather than just warning and returning NULL?
> 
> I know this is generally a bad thing, but in this case, when a debug
> option has been enabled exactly for this purpose, I think it is not
> inappropriate to BUG() when encountering such a mapping. But I am
> happy to relax it to a WARN() and return NULL instead, but in that
> case, it should be unconditional imo and not based on
> CONFIG_DEBUG_VIRTUAL or the likes.

Sounds sane to me.

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