Re: [PATCH v2 13/15] mm: support GUP-triggered unsharing of anonymous pages

[David Hildenbrand]

On 21.03.22 17:18, Jason Gunthorpe wrote:
> On Mon, Mar 21, 2022 at 05:15:06PM +0100, David Hildenbrand wrote:
>> On 19.03.22 00:30, Jason Gunthorpe wrote:
>>> On Tue, Mar 15, 2022 at 11:47:39AM +0100, David Hildenbrand wrote:
>>>> Whenever GUP currently ends up taking a R/O pin on an anonymous page that
>>>> might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault
>>>> on the page table entry will end up replacing the mapped anonymous page
>>>> due to COW, resulting in the GUP pin no longer being consistent with the
>>>> page actually mapped into the page table.
>>>>
>>>> The possible ways to deal with this situation are:
>>>>  (1) Ignore and pin -- what we do right now.
>>>>  (2) Fail to pin -- which would be rather surprising to callers and
>>>>      could break user space.
>>>>  (3) Trigger unsharing and pin the now exclusive page -- reliable R/O
>>>>      pins.
>>>>
>>>> We want to implement 3) because it provides the clearest semantics and
>>>> allows for checking in unpin_user_pages() and friends for possible BUGs:
>>>> when trying to unpin a page that's no longer exclusive, clearly
>>>> something went very wrong and might result in memory corruptions that
>>>> might be hard to debug. So we better have a nice way to spot such
>>>> issues.
>>>>
>>>> To implement 3), we need a way for GUP to trigger unsharing:
>>>> FAULT_FLAG_UNSHARE. FAULT_FLAG_UNSHARE is only applicable to R/O mapped
>>>> anonymous pages and resembles COW logic during a write fault. However, in
>>>> contrast to a write fault, GUP-triggered unsharing will, for example, still
>>>> maintain the write protection.
>>>
>>> Given the way this series has developed you might want to call this
>>> FAULT_FLAG_MAKE_ANON_EXCLUSIVE
>>>
>>> Which strikes me as more directly connected to what it is trying to
>>> do.
>>
>> I thought about something similar along those lines, and I think it
>> would apply even when extending that mechanism to anything !anon inside
>> a MAP_PRIVATE mapping.
>>
>> The whole
>>
>> const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;
> 
> I think the extra words are worthwhile, share makes me think about
> MAP_SHARED as we don't really use shared anywhere else FWICT..

Yeah, my point would be that you can only "unshare" in MAP_PRIVATE
(!MAP_SHARED) :)

-- 
Thanks,

David / dhildenb