Commit 96312e61 authored Mar 09, 2018 by Andrea Arcangeli Committed by Linus Torvalds Mar 09, 2018

mm/gup.c: teach get_user_pages_unlocked to handle FOLL_NOWAIT

KVM is hanging during postcopy live migration with userfaultfd because
get_user_pages_unlocked is not capable to handle FOLL_NOWAIT.

Earlier FOLL_NOWAIT was only ever passed to get_user_pages.

Specifically faultin_page (the callee of get_user_pages_unlocked caller)
doesn't know that if FAULT_FLAG_RETRY_NOWAIT was set in the page fault
flags, when VM_FAULT_RETRY is returned, the mmap_sem wasn't actually
released (even if nonblocking is not NULL). So it sets *nonblocking to
zero and the caller won't release the mmap_sem thinking it was already
released, but it wasn't because of FOLL_NOWAIT.

Link: http://lkml.kernel.org/r/20180302174343.5421-2-aarcange@redhat.com

Fixes: ce53053c ("kvm: switch get_user_page_nowait() to get_user_pages_unlocked()")
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Tested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 1b4cfe3c

mm/gup.c

+5 −2

Original line number	Diff line number	Diff line
		@@ -516,7 +516,7 @@ static int faultin_page(struct task_struct tsk, struct vm_area_struct vma,
		}

		if (ret & VM_FAULT_RETRY) {
		if (nonblocking)
		if (nonblocking && !(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
		*nonblocking = 0;
		return -EBUSY;
		}
		@@ -890,7 +890,10 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
		break;
		}
		if (*locked) {
		/* VM_FAULT_RETRY didn't trigger */
		/*
		* VM_FAULT_RETRY didn't trigger or it was a
		* FOLL_NOWAIT.
		*/
		if (!pages_done)
		pages_done = ret;
		break;