mm/gup: remove (VM_)BUG_ONs

Especially once we hit one of the assertions in
sanity_check_pinned_pages(), observing follow-up assertions failing in
other code can give good clues about what went wrong, so use
VM_WARN_ON_ONCE instead.

While at it, let's just convert all VM_BUG_ON to VM_WARN_ON_ONCE as well.
Add one comment for the pfn_valid() check.

We have to introduce VM_WARN_ON_ONCE_VMA() to make that fly.

Drop the BUG_ON after mmap_read_lock_killable(), if that ever returns
something > 0 we're in bigger trouble. Convert the other BUG_ON's into
VM_WARN_ON_ONCE as well, they are in a similar domain "should never
happen", but more reasonable to check for during early testing.

[[e-mail]: use the _FOLIO variant where possible, per Lorenzo]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: David Hildenbrand <[e-mail]>
Acked-by: Vlastimil Babka <[e-mail]>
Reviewed-by: Suren Baghdasaryan <[e-mail]>
Reviewed-by: Lorenzo Stoakes <[e-mail]>
Acked-by: SeongJae Park <[e-mail]>
Reviewed-by: Liam R. Howlett <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Reviewed-by: John Hubbard <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Cc: Peter Xu <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/mmdebug.h

···
 	}								\
 	unlikely(__ret_warn_once);					\
 })
+#define VM_WARN_ON_ONCE_VMA(cond, vma)		({			\
+	static bool __section(".data..once") __warned;			\
+	int __ret_warn_once = !!(cond);					\
+									\
+	if (unlikely(__ret_warn_once && !__warned)) {			\
+		dump_vma(vma);						\
+		__warned = true;					\
+		WARN_ON(1);						\
+	}								\
+	unlikely(__ret_warn_once);					\
+})
 #define VM_WARN_ON_VMG(cond, vmg)		({			\
 	int __ret_warn = !!(cond);					\
 									\
···
 #define VM_WARN_ON_FOLIO(cond, folio)  BUILD_BUG_ON_INVALID(cond)
 #define VM_WARN_ON_ONCE_FOLIO(cond, folio)  BUILD_BUG_ON_INVALID(cond)
 #define VM_WARN_ON_ONCE_MM(cond, mm)  BUILD_BUG_ON_INVALID(cond)
+#define VM_WARN_ON_ONCE_VMA(cond, vma)  BUILD_BUG_ON_INVALID(cond)
 #define VM_WARN_ON_VMG(cond, vmg)  BUILD_BUG_ON_INVALID(cond)
 #define VM_WARN_ONCE(cond, format...) BUILD_BUG_ON_INVALID(cond)
 #define VM_WARN(cond, format...) BUILD_BUG_ON_INVALID(cond)

mm/gup.c

···
 		    !folio_test_anon(folio))
 			continue;
 		if (!folio_test_large(folio) || folio_test_hugetlb(folio))
-			VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page), page);
+			VM_WARN_ON_ONCE_FOLIO(!PageAnonExclusive(&folio->page), folio);
 		else
 			/* Either a PTE-mapped or a PMD-mapped THP. */
-			VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page) &&
-				       !PageAnonExclusive(page), page);
+			VM_WARN_ON_ONCE_PAGE(!PageAnonExclusive(&folio->page) &&
+					     !PageAnonExclusive(page), page);
 	}
 }
 
···
 	if (!pmd_write(pmdval) && gup_must_unshare(vma, flags, page))
 		return ERR_PTR(-EMLINK);
 
-	VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
-			!PageAnonExclusive(page), page);
+	VM_WARN_ON_ONCE_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
+			     !PageAnonExclusive(page), page);
 
 	ret = try_grab_folio(page_folio(page), 1, flags);
 	if (ret)
···
 		goto out;
 	}
 
-	VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
-		       !PageAnonExclusive(page), page);
+	VM_WARN_ON_ONCE_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
+			     !PageAnonExclusive(page), page);
 
 	/* try_grab_folio() does nothing unless FOLL_GET or FOLL_PIN is set. */
 	ret = try_grab_folio(folio, 1, flags);
···
 	if (unshare) {
 		fault_flags |= FAULT_FLAG_UNSHARE;
 		/* FAULT_FLAG_WRITE and FAULT_FLAG_UNSHARE are incompatible */
-		VM_BUG_ON(fault_flags & FAULT_FLAG_WRITE);
+		VM_WARN_ON_ONCE(fault_flags & FAULT_FLAG_WRITE);
 	}
 
 	ret = handle_mm_fault(vma, address, fault_flags, NULL);
···
 		}
 
 		/* VM_FAULT_RETRY or VM_FAULT_COMPLETED cannot return errors */
-		if (!*locked) {
-			BUG_ON(ret < 0);
-			BUG_ON(ret >= nr_pages);
-		}
+		VM_WARN_ON_ONCE(!*locked && (ret < 0 || ret >= nr_pages));
 
 		if (ret > 0) {
 			nr_pages -= ret;
···
 
 		ret = mmap_read_lock_killable(mm);
 		if (ret) {
-			BUG_ON(ret > 0);
 			if (!pages_done)
 				pages_done = ret;
 			break;
···
 				       pages, locked);
 		if (!*locked) {
 			/* Continue to retry until we succeeded */
-			BUG_ON(ret != 0);
+			VM_WARN_ON_ONCE(ret != 0);
 			goto retry;
 		}
 		if (ret != 1) {
-			BUG_ON(ret > 1);
+			VM_WARN_ON_ONCE(ret > 1);
 			if (!pages_done)
 				pages_done = ret;
 			break;
···
 	int gup_flags;
 	long ret;
 
-	VM_BUG_ON(!PAGE_ALIGNED(start));
-	VM_BUG_ON(!PAGE_ALIGNED(end));
-	VM_BUG_ON_VMA(start < vma->vm_start, vma);
-	VM_BUG_ON_VMA(end   > vma->vm_end, vma);
+	VM_WARN_ON_ONCE(!PAGE_ALIGNED(start));
+	VM_WARN_ON_ONCE(!PAGE_ALIGNED(end));
+	VM_WARN_ON_ONCE_VMA(start < vma->vm_start, vma);
+	VM_WARN_ON_ONCE_VMA(end   > vma->vm_end, vma);
 	mmap_assert_locked(mm);
 
 	/*
···
 	int gup_flags;
 	long ret;
 
-	VM_BUG_ON(!PAGE_ALIGNED(start));
-	VM_BUG_ON(!PAGE_ALIGNED(end));
+	VM_WARN_ON_ONCE(!PAGE_ALIGNED(start));
+	VM_WARN_ON_ONCE(!PAGE_ALIGNED(end));
 	mmap_assert_locked(mm);
 
 	/*
···
 		} else if (pte_special(pte))
 			goto pte_unmap;
 
-		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
+		/* If it's not marked as special it must have a valid memmap. */
+		VM_WARN_ON_ONCE(!pfn_valid(pte_pfn(pte)));
 		page = pte_page(pte);
 
 		folio = try_grab_folio_fast(page, 1, flags);