mm: Don't pin ZERO_PAGE in pin_user_pages()

Make pin_user_pages*() leave a ZERO_PAGE unpinned if it extracts a pointer
to it from the page tables and make unpin_user_page*() correspondingly
ignore a ZERO_PAGE when unpinning.  We don't want to risk overrunning a
zero page's refcount as we're only allowed ~2 million pins on it -
something that userspace can conceivably trigger.

Add a pair of functions to test whether a page or a folio is a ZERO_PAGE.

Signed-off-by: David Howells <[email protected]>
cc: Christoph Hellwig <[email protected]>
cc: David Hildenbrand <[email protected]>
cc: Lorenzo Stoakes <[email protected]>
cc: Andrew Morton <[email protected]>
cc: Jens Axboe <[email protected]>
cc: Al Viro <[email protected]>
cc: Matthew Wilcox <[email protected]>
cc: Jan Kara <[email protected]>
cc: Jeff Layton <[email protected]>
cc: Jason Gunthorpe <[email protected]>
cc: Logan Gunthorpe <[email protected]>
cc: Hillf Danton <[email protected]>
cc: Christian Brauner <[email protected]>
cc: Linus Torvalds <[email protected]>
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
Reviewed-by: Lorenzo Stoakes <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

0 files changed, 0 insertions, 0 deletions