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.
