On Fri, Jan 27, 2023 at 12:52:38AM +0000, Al Viro wrote:

On Thu, Jan 26, 2023 at 11:56:50PM +0000, David Howells wrote:

quoted

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).

Umm...  OK, I really need to reread that area.  Hopefully will be done with
that by tomorrow...