Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)

[David Hildenbrand]

On 17.12.21 21:36, Linus Torvalds wrote:
> On Fri, Dec 17, 2021 at 12:18 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>> On 17.12.21 20:22, Linus Torvalds wrote:
>>> On Fri, Dec 17, 2021 at 11:04 AM Linus Torvalds
>>> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>>>
>>>  - get a "readonly" copy of a local private page using FAULT_FLAG_UNSHARE.
>>>
>>>    This just increments the page count, because mapcount == 1.
>>>
>>>  - fork()
>>>
>>>  - unmap in the original
>>>
>>>  - child now has "mapcount == 1" on a page again, but refcount is
>>> elevated, and child HAS TO COW before writing.
>>
>> Hi Linus,
>>
>> This is just GUP before fork(), which is in general
>> problematic/incompatible with sharing.
> 
> Note that my example was not meant to be an example of a problem per
> se, but purely as an example of how meaningless 'mapcount' is, and how
> 'mapcount==1' isn't really a very meaningful test.
> 
> So it wasn't mean to show "look, GUP before fork is problematic".  We
> have that problem already solved at least for regular pages.
> 
> It was purely meant to show how "mapcount==1" isn't a meaningful thing
> to test, and my worry about how you're adding that nonsensical test to
> the new code.
> 
>> Let's just take a look at what refcount does *wrong*. Let's use an
>> adjusted version of your example above, because it's a perfect fit:
>>
>> 1. mem = mmap(pagesize, MAP_PRIVATE)
>> -> refcount == 1
>>
>> 2. memset(mem, 0, pagesize); /* Page is mapped R/W */
>>
>> 3. fork() /* Page gets mapped R/O */
>> -> refcount > 1
>>
>> 4. child quits
>> -> refcount == 1
>>
>> 5. Take a R/O pin (RDMA, VFIO, ...)
>> -> refcount > 1
>>
>> 6. memset(mem, 0xff, pagesize);
>> -> Write fault -> COW
> 
> I do not believe this is actually a bug.

It's debatable if it's a BUG or not (I think it is one). It's for sure
inconsistent.

> 
> You asked for a R/O pin, and you got one.
> 
> Then somebody else modified that page, and you got exactly what you
> asked for - a COW event. The original R/O pin has the original page
> that it asked for, and can read it just fine.

Where in the code did I ask for a COW event? I asked for a R/O pin, not
any kind of memory protection.

-- 
Thanks,

David / dhildenb