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

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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?


thanks,
--
John Hubbard
NVIDIA



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