lazy-pages: don't try to uffd_copy to removed memory regions

It is possible that when pages request from the remove source arrive, part
of the memory range covered by the request would be already gone because of
madvise(MADV_DONTNEED), mremap() etc.
Ensure we are not trying to uffd_copy more than we are allowed.
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/uffd.c

@@ -839,6 +839,7 @@ static int uffd_io_complete(struct page_read *pr, unsigned long img_addr, int nr
 {
 	struct lazy_pages_info *lpi;
 	unsigned long addr = 0;
+	int req_pages;
 	struct lazy_iov *req;
 
 	lpi = container_of(pr, struct lazy_pages_info, pr);
@@ -864,6 +865,14 @@ static int uffd_io_complete(struct page_read *pr, unsigned long img_addr, int nr
 	if (!addr)
 		return 0;
 
+	/*
+	 * by the time we get the pages from the remote source, parts
+	 * of the request may already be gone because of
+	 * UFFD_EVENT_{REMAP,REMOVE,UNMAP}
+	 */
+	req_pages = (req->end - req->start) / PAGE_SIZE;
+	nr = min(nr, req_pages);
+
 	return uffd_copy(lpi, addr, nr);
 }