On Thu, Jan 26, 2023 at 11:56:50PM +0000, David Howells wrote: > Al says that pinning a page (ie. FOLL_PIN) could cause a deadlock if a page is > vmspliced into a pipe with the pipe holding a pin on it because pinned pages > are removed from all page tables. Is this actually the case? I can't see > offhand where in mm/gup.c it does this. It doesn't; sorry, really confused memories of what's going on, took a while to sort them out (FWIW, writeback is where we unmap and check if page is pinned, while pin_user_pages running into an unmapped page will end up with handle_mm_fault() (->fault(), actually) try to get the sucker locked and block on that until the writeback is over). Said that, I still think that pinned pages (arbitrary pagecache ones, at that) ending up in a pipe is a seriously bad idea. It's trivial to arrange for them to stay that way indefinitely - no priveleges needed, very few limits, etc.
