Re: [RFC 3/8] mm: Avoid using set_page_count() in set_page_recounted()

[John Hubbard <jhubbard@nvidia.com>]

On 10/26/21 11:21, Pasha Tatashin wrote:
> It must return the same thing, if it does not we have a bug in our
> kernel which may lead to memory corruptions and security holes.
> 
> So today we have this:
>     VM_BUG_ON_PAGE(page_ref_count(page), page); -> check ref_count is 0
>     < What if something modified here? Hmm..>
>     set_page_count(page, 1); -> Yet we reset it to 1.
> 
> With my proposed change:
>     VM_BUG_ON_PAGE(page_ref_count(page), page); -> check ref_count is 0
>     refcnt = page_ref_inc_return(page);  -> ref_count better be 1.
>     VM_BUG_ON_PAGE(refcnt != 1, page); -> Verify that it is 1.
> 

Yes, you are just repeating what the diffs say.

But it's still not good to have this function name doing something completely
different than its name indicates.

>>
>> I understand where this patchset is going, but this intermediate step is
>> not a good move.
>>
>> Also, for the overall series, if you want to change from
>> "set_page_count()" to "inc_and_verify_val_equals_one()", then the way to
>> do that is *not* to depend solely on VM_BUG*() to verify. Instead,
>> return something like -EBUSY if incrementing the value results in a
>> surprise, and let the caller decide how to handle it.
> 
> Actually, -EBUSY would be OK if the problems were because we failed to
> modify refcount for some reason, but if we modified refcount and got
> an unexpected value (i.e underflow/overflow) we better report it right
> away instead of waiting for memory corruption to happen.
> 

Having the caller do the BUG() or VM_BUG*() is not a significant delay.


thanks,
-- 
John Hubbard
NVIDIA