Re: [PATCHv3 4/5] mm: make compound_head() robust

[Vlastimil Babka <vbabka@suse.cz>]

On 08/26/2015 05:04 PM, Kirill A. Shutemov wrote:
>>>                                                                          That's
>>> bad news then. It's not that we would trigger that bit when the rcu_head part of
>>> the union is "active". It's that pfn scanners could inspect such page at
>>> arbitrary time, see the bit 0 set (due to RCU processing) and think that it's a
>>> tail page of a compound page, and interpret the rest of the pointer as a pointer
>>> to the head page (to test it for flags etc).
>>
>> On the other hand, if you avoid scanning rcu_head structures for pages
>> that are currently waiting for a grace period, no problem.  RCU does
>> not use the rcu_head structure at all except for during the time between
>> when call_rcu() is invoked on that rcu_head structure and the time that
>> the callback is invoked.
>>
>> Is there some other page state that indicates that the page is waiting
>> for a grace period?  If so, you could simply avoid testing that bit in
>> that case.
>
> No, I don't think so.
>
> For compound pages most of info of its state is stored in head page (e.g.
> page_count(), flags, etc). So if we examine random page (pfn scanner case)
> the very first thing we want to know if we stepped on tail page.
> PageTail() is what I wanted to encode in the bit...
>
> What if we change order of fields within rcu_head and put ->func first?

Or change the order of compound_head wrt the rest?

> Can we expect this pointer to have bit 0 always clear?

That's probably a question whether $compiler is guaranteed to align 
functions on all architectures...

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