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

[John Hubbard <jhubbard@nvidia.com>]

On 10/27/21 11:27, Pasha Tatashin wrote:
> On Wed, Oct 27, 2021 at 1:12 AM John Hubbard <jhubbard@nvidia.com> wrote:
>>
>> 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 see, I can rename it to: 'set_page_recounted/get_page_recounted' ?
> 

What? No, that's not where I was going at all. The function is already
named set_page_refcounted(), and one of the problems I see is that your
changes turn it into something that most certainly does not
set_page_refounted(). Instead, this patch *increments* the refcount.
That is not the same thing.

And then it uses a .config-sensitive assertion to "prevent" problems.
And by that I mean, the wording throughout this series seems to equate
VM_BUG_ON_PAGE() assertions with real assertions. They are only active,
however, in CONFIG_DEBUG_VM configurations, and provide no protection at
all for normal (most distros) users. That's something that the wording,
comments, and even design should be tweaked to account for.


>>
>>>>
>>>> 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.
> 
> We cannot guarantee that new callers in the future will check return
> values, the idea behind this work is to ensure that we are always
> protected from refcount underflow/overflow and invalid refcount
> modifications by set_refcount.
> 

I don't have a problem with putting assertions closest to where they should
fire. That's a good thing. I'm looking here for ways to fix up the problems
listed in the points above, though.

And I do want to point out another thing, though, and that is: generally, we
don't have to program to quite the level of defensiveness you seem to be at.
If return values must be checked, they usually are in the kernel--and we even
have tooling to enforce it:

/*
  *   gcc: 
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-warn_005funused_005fresult-function-attribute
  * clang: https://clang.llvm.org/docs/AttributeReference.html#nodiscard-warn-unused-result
  */
#define __must_check                    __attribute__((__warn_unused_result__))


Please take that into consideration when weighing tradeoffs, just sort of in
general.



thanks,
-- 
John Hubbard
NVIDIA