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.
