Re: [RFCv2 08/16] KVM: Use GUP instead of copy_from/to_user() to access guest memory

[John Hubbard <jhubbard@nvidia.com>]

On 10/25/20 9:21 PM, Matthew Wilcox wrote:
> On Thu, Oct 22, 2020 at 12:58:14PM -0700, John Hubbard wrote:
>> On 10/22/20 4:49 AM, Matthew Wilcox wrote:
>>> On Tue, Oct 20, 2020 at 01:25:59AM -0700, John Hubbard wrote:
>>>> Should copy_to_guest() use pin_user_pages_unlocked() instead of gup_unlocked?
>>>> We wrote a  "Case 5" in Documentation/core-api/pin_user_pages.rst, just for this
>>>> situation, I think:
>>>>
>>>>
>>>> CASE 5: Pinning in order to write to the data within the page
>>>> -------------------------------------------------------------
>>>> Even though neither DMA nor Direct IO is involved, just a simple case of "pin,
>>>> write to a page's data, unpin" can cause a problem. Case 5 may be considered a
>>>> superset of Case 1, plus Case 2, plus anything that invokes that pattern. In
>>>> other words, if the code is neither Case 1 nor Case 2, it may still require
>>>> FOLL_PIN, for patterns like this:
>>>>
>>>> Correct (uses FOLL_PIN calls):
>>>>       pin_user_pages()
>>>>       write to the data within the pages
>>>>       unpin_user_pages()
>>>
>>> Case 5 is crap though.  That bug should have been fixed by getting
>>> the locking right.  ie:
>>>
>>> 	get_user_pages_fast();
>>> 	lock_page();
>>> 	kmap();
>>> 	set_bit();
>>> 	kunmap();
>>> 	set_page_dirty()
>>> 	unlock_page();
>>>
>>> I should have vetoed that patch at the time, but I was busy with other things.
>>
>> It does seem like lock_page() is better, for now at least, because it
>> forces the kind of synchronization with file system writeback that is
>> still yet to be implemented for pin_user_pages().
>>
>> Long term though, Case 5 provides an alternative way to do this
>> pattern--without using lock_page(). Also, note that Case 5, *in
>> general*, need not be done page-at-a-time, unlike the lock_page()
>> approach. Therefore, Case 5 might potentially help at some call sites,
>> either for deadlock avoidance or performance improvements.
>>
>> In other words, once the other half of the pin_user_pages() plan is
>> implemented, either of these approaches should work.
>>
>> Or, are you thinking that there is never a situation in which Case 5 is
>> valid?
> 
> I don't think the page pinning approach is ever valid.  For file

Could you qualify that? Surely you don't mean that the entire pin_user_pages
story is a waste of time--I would have expected you to make more noise
earlier if you thought that, yes?

> mappings, the infiniband registration should take a lease on the inode,
> blocking truncation.  DAX shouldn't be using struct pages at all, so
> there shouldn't be anything there to pin.
> 
> It's been five years since DAX was merged, and page pinning still
> doesn't work.  How much longer before the people who are pushing it
> realise that it's fundamentally flawed?
> 

Is this a separate rant about *only* DAX, or is general RDMA in your sights
too? :)



thanks,
-- 
John Hubbard
NVIDIA