On Thu, Feb 10, 2022 at 11:20:31AM -0800, John Hubbard wrote: > On 2/10/22 11:17, Minchan Kim wrote: > > * pin_user_pages() - pin user pages in memory for use by other devices > > < snip > > > * @pages: array that receives pointers to the pages pinned. > > * Should be at least nr_pages long. Or NULL, if caller > > * only intends to ensure the pages are faulted in. > > > > pin_user_pages(,, pages = NULL, ); > > gup_flags |= FOLL_PIN > > __get_user_pages_locked > > __get_user_pages > > .. > > VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN))); > > Only FOLL_GET or FOLL_PIN are supposed to fill in the **pages array. So > if a caller passes a null **pages arg, then that caller must not also > set FOLL_GET or FOLL_PIN. That's what the VM_BUG_ON() is expressing. Yub, but pin_user_pages adds FOLL_PIN unconditinally and the comments says it supports NUU pages argument. Isn't it conflict? > > Perhaps that should be part of the documentation. It sort of is already, > for get_user_pages*(). I expected it was just copied from get_user_pages.
