mm: return an ERR_PTR from __filemap_get_folio

Instead of returning NULL for all errors, distinguish between:

 - no entry found and not asked to allocated (-ENOENT)
 - failed to allocate memory (-ENOMEM)
 - would block (-EAGAIN)

so that callers don't have to guess the error based on the passed in
flags.

Also pass through the error through the direct callers: filemap_get_folio,
filemap_lock_folio filemap_grab_folio and filemap_get_incore_folio.

[hch@lst.de: fix null-pointer deref]
  Link: https://lkml.kernel.org/r/20230310070023.GA13563@lst.de
  Link: https://lkml.kernel.org/r/20230310043137.GA1624890@u2004
Link: https://lkml.kernel.org/r/20230307143410.28031-8-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Ryusuke Konishi <konishi.ryusuke@gmail.com> [nilfs2]
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/afs/dir.c

@@ -319,16 +319,16 @@ static struct afs_read *afs_read_dir(struct afs_vnode *dvnode, struct key *key)
 		struct folio *folio;
 
 		folio = filemap_get_folio(mapping, i);
-		if (!folio) {
+		if (IS_ERR(folio)) {
 			if (test_and_clear_bit(AFS_VNODE_DIR_VALID, &dvnode->flags))
 				afs_stat_v(dvnode, n_inval);
-
-			ret = -ENOMEM;
 			folio = __filemap_get_folio(mapping,
 						    i, FGP_LOCK | FGP_CREAT,
 						    mapping->gfp_mask);
-			if (!folio)
+			if (IS_ERR(folio)) {
+				ret = PTR_ERR(folio);
 				goto error;
+			}
 			folio_attach_private(folio, (void *)1);
 			folio_unlock(folio);
 		}
@@ -524,7 +524,7 @@ static int afs_dir_iterate(struct inode *dir, struct dir_context *ctx,
 		 */
 		folio = __filemap_get_folio(dir->i_mapping, ctx->pos / PAGE_SIZE,
 					    FGP_ACCESSED, 0);
-		if (!folio) {
+		if (IS_ERR(folio)) {
 			ret = afs_bad(dvnode, afs_file_error_dir_missing_page);
 			break;
 		}

fs/afs/dir_edit.c

@@ -115,7 +115,7 @@ static struct folio *afs_dir_get_folio(struct afs_vnode *vnode, pgoff_t index)
 	folio = __filemap_get_folio(mapping, index,
 				    FGP_LOCK | FGP_ACCESSED | FGP_CREAT,
 				    mapping->gfp_mask);
-	if (!folio)
+	if (IS_ERR(folio))
 		clear_bit(AFS_VNODE_DIR_VALID, &vnode->flags);
 	else if (folio && !folio_test_private(folio))
 		folio_attach_private(folio, (void *)1);

fs/afs/write.c

@@ -232,7 +232,7 @@ static void afs_kill_pages(struct address_space *mapping,
 		_debug("kill %lx (to %lx)", index, last);
 
 		folio = filemap_get_folio(mapping, index);
-		if (!folio) {
+		if (IS_ERR(folio)) {
 			next = index + 1;
 			continue;
 		}
@@ -270,7 +270,7 @@ static void afs_redirty_pages(struct writeback_control *wbc,
 		_debug("redirty %llx @%llx", len, start);
 
 		folio = filemap_get_folio(mapping, index);
-		if (!folio) {
+		if (IS_ERR(folio)) {
 			next = index + 1;
 			continue;
 		}

fs/ext4/inode.c

@@ -5395,7 +5395,7 @@ static void ext4_wait_for_tail_page_commit(struct inode *inode)
 	while (1) {
 		struct folio *folio = filemap_lock_folio(inode->i_mapping,
 				      inode->i_size >> PAGE_SHIFT);
-		if (!folio)
+		if (IS_ERR(folio))
 			return;
 		ret = __ext4_journalled_invalidate_folio(folio, offset,
 						folio_size(folio) - offset);

fs/ext4/move_extent.c

@@ -141,18 +141,18 @@ mext_folio_double_lock(struct inode *inode1, struct inode *inode2,
 	flags = memalloc_nofs_save();
 	folio[0] = __filemap_get_folio(mapping[0], index1, fgp_flags,
 			mapping_gfp_mask(mapping[0]));
-	if (!folio[0]) {
+	if (IS_ERR(folio[0])) {
 		memalloc_nofs_restore(flags);
-		return -ENOMEM;
+		return PTR_ERR(folio[0]);
 	}
 
 	folio[1] = __filemap_get_folio(mapping[1], index2, fgp_flags,
 			mapping_gfp_mask(mapping[1]));
 	memalloc_nofs_restore(flags);
-	if (!folio[1]) {
+	if (IS_ERR(folio[1])) {
 		folio_unlock(folio[0]);
 		folio_put(folio[0]);
-		return -ENOMEM;
+		return PTR_ERR(folio[1]);
 	}
 	/*
 	 * __filemap_get_folio() may not wait on folio's writeback if

fs/hugetlbfs/inode.c

@@ -697,7 +697,7 @@ static void hugetlbfs_zero_partial_page(struct hstate *h,
 	struct folio *folio;
 
 	folio = filemap_lock_folio(mapping, idx);
-	if (!folio)
+	if (IS_ERR(folio))
 		return;
 
 	start = start & ~huge_page_mask(h);

fs/iomap/buffered-io.c

@@ -468,19 +468,12 @@ EXPORT_SYMBOL_GPL(iomap_is_partially_uptodate);
 struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos)
 {
 	unsigned fgp = FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE | FGP_NOFS;
-	struct folio *folio;
 
 	if (iter->flags & IOMAP_NOWAIT)
 		fgp |= FGP_NOWAIT;
 
-	folio = __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
+	return __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
 			fgp, mapping_gfp_mask(iter->inode->i_mapping));
-	if (folio)
-		return folio;
-
-	if (iter->flags & IOMAP_NOWAIT)
-		return ERR_PTR(-EAGAIN);
-	return ERR_PTR(-ENOMEM);
 }
 EXPORT_SYMBOL_GPL(iomap_get_folio);
 
@@ -911,7 +904,7 @@ static int iomap_write_delalloc_scan(struct inode *inode,
 		/* grab locked page */
 		folio = filemap_lock_folio(inode->i_mapping,
 				start_byte >> PAGE_SHIFT);
-		if (!folio) {
+		if (IS_ERR(folio)) {
 			start_byte = ALIGN_DOWN(start_byte, PAGE_SIZE) +
 					PAGE_SIZE;
 			continue;

fs/netfs/buffered_read.c

@@ -350,8 +350,8 @@ int netfs_write_begin(struct netfs_inode *ctx,
 retry:
 	folio = __filemap_get_folio(mapping, index, fgp_flags,
 				    mapping_gfp_mask(mapping));
-	if (!folio)
-		return -ENOMEM;
+	if (IS_ERR(folio))
+		return PTR_ERR(folio);
 
 	if (ctx->ops->check_write_begin) {
 		/* Allow the netfs (eg. ceph) to flush conflicts. */

fs/nfs/file.c

@@ -336,8 +336,8 @@ static int nfs_write_begin(struct file *file, struct address_space *mapping,
 
 start:
 	folio = nfs_folio_grab_cache_write_begin(mapping, pos >> PAGE_SHIFT);
-	if (!folio)
-		return -ENOMEM;
+	if (IS_ERR(folio))
+		return PTR_ERR(folio);
 	*pagep = &folio->page;
 
 	ret = nfs_flush_incompatible(file, folio);

fs/nilfs2/page.c

@@ -259,10 +259,10 @@ int nilfs_copy_dirty_pages(struct address_space *dmap,
 			NILFS_PAGE_BUG(&folio->page, "inconsistent dirty state");
 
 		dfolio = filemap_grab_folio(dmap, folio->index);
-		if (unlikely(!dfolio)) {
+		if (unlikely(IS_ERR(dfolio))) {
 			/* No empty page is added to the page cache */
-			err = -ENOMEM;
 			folio_unlock(folio);
+			err = PTR_ERR(dfolio);
 			break;
 		}
 		if (unlikely(!folio_buffers(folio)))
@@ -311,7 +311,7 @@ void nilfs_copy_back_pages(struct address_space *dmap,
 
 		folio_lock(folio);
 		dfolio = filemap_lock_folio(dmap, index);
-		if (dfolio) {
+		if (!IS_ERR(dfolio)) {
 			/* overwrite existing folio in the destination cache */
 			WARN_ON(folio_test_dirty(dfolio));
 			nilfs_copy_page(&dfolio->page, &folio->page, 0);

include/linux/pagemap.h

@@ -520,7 +520,8 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
  * Looks up the page cache entry at @mapping & @index.  If a folio is
  * present, it is returned with an increased refcount.
  *
- * Otherwise, %NULL is returned.
+ * Return: A folio or ERR_PTR(-ENOENT) if there is no folio in the cache for
+ * this index.  Will not return a shadow, swap or DAX entry.
  */
 static inline struct folio *filemap_get_folio(struct address_space *mapping,
 					pgoff_t index)
@@ -537,8 +538,8 @@ static inline struct folio *filemap_get_folio(struct address_space *mapping,
  * present, it is returned locked with an increased refcount.
  *
  * Context: May sleep.
- * Return: A folio or %NULL if there is no folio in the cache for this
- * index.  Will not return a shadow, swap or DAX entry.
+ * Return: A folio or ERR_PTR(-ENOENT) if there is no folio in the cache for
+ * this index.  Will not return a shadow, swap or DAX entry.
  */
 static inline struct folio *filemap_lock_folio(struct address_space *mapping,
 					pgoff_t index)
@@ -555,8 +556,8 @@ static inline struct folio *filemap_lock_folio(struct address_space *mapping,
  * a new folio is created. The folio is locked, marked as accessed, and
  * returned.
  *
- * Return: A found or created folio. NULL if no folio is found and failed to
- * create a folio.
+ * Return: A found or created folio. ERR_PTR(-ENOMEM) if no folio is found
+ * and failed to create a folio.
  */
 static inline struct folio *filemap_grab_folio(struct address_space *mapping,
 					pgoff_t index)

mm/filemap.c

@@ -1907,7 +1907,7 @@ void *filemap_get_entry(struct address_space *mapping, pgoff_t index)
  *
  * If there is a page cache page, it is returned with an increased refcount.
  *
- * Return: The found folio or %NULL otherwise.
+ * Return: The found folio or an ERR_PTR() otherwise.
  */
 struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 		int fgp_flags, gfp_t gfp)
@@ -1925,7 +1925,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 		if (fgp_flags & FGP_NOWAIT) {
 			if (!folio_trylock(folio)) {
 				folio_put(folio);
-				return NULL;
+				return ERR_PTR(-EAGAIN);
 			}
 		} else {
 			folio_lock(folio);
@@ -1964,7 +1964,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 
 		folio = filemap_alloc_folio(gfp, 0);
 		if (!folio)
-			return NULL;
+			return ERR_PTR(-ENOMEM);
 
 		if (WARN_ON_ONCE(!(fgp_flags & (FGP_LOCK | FGP_FOR_MMAP))))
 			fgp_flags |= FGP_LOCK;
@@ -1989,6 +1989,8 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 			folio_unlock(folio);
 	}
 
+	if (!folio)
+		return ERR_PTR(-ENOENT);
 	return folio;
 }
 EXPORT_SYMBOL(__filemap_get_folio);
@@ -3258,7 +3260,7 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
 	 * Do we have something in the page cache already?
 	 */
 	folio = filemap_get_folio(mapping, index);
-	if (likely(folio)) {
+	if (likely(!IS_ERR(folio))) {
 		/*
 		 * We found the page, so try async readahead before waiting for
 		 * the lock.
@@ -3287,7 +3289,7 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
 		folio = __filemap_get_folio(mapping, index,
 					  FGP_CREAT|FGP_FOR_MMAP,
 					  vmf->gfp_mask);
-		if (!folio) {
+		if (IS_ERR(folio)) {
 			if (fpin)
 				goto out_retry;
 			filemap_invalidate_unlock_shared(mapping);
@@ -3638,7 +3640,7 @@ static struct folio *do_read_cache_folio(struct address_space *mapping,
 		filler = mapping->a_ops->read_folio;
 repeat:
 	folio = filemap_get_folio(mapping, index);
-	if (!folio) {
+	if (IS_ERR(folio)) {
 		folio = filemap_alloc_folio(gfp, 0);
 		if (!folio)
 			return ERR_PTR(-ENOMEM);

mm/folio-compat.c

@@ -97,7 +97,7 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
 	struct folio *folio;
 
 	folio = __filemap_get_folio(mapping, index, fgp_flags, gfp);
-	if (!folio)
+	if (IS_ERR(folio))
 		return NULL;
 	return folio_file_page(folio, index);
 }

mm/huge_memory.c

@@ -3092,7 +3092,7 @@ static int split_huge_pages_in_file(const char *file_path, pgoff_t off_start,
 		struct folio *folio = filemap_get_folio(mapping, index);
 
 		nr_pages = 1;
-		if (!folio)
+		if (IS_ERR(folio))
 			continue;
 
 		if (!folio_test_large(folio))

mm/hugetlb.c

@@ -5780,7 +5780,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 	 */
 	new_folio = false;
 	folio = filemap_lock_folio(mapping, idx);
-	if (!folio) {
+	if (IS_ERR(folio)) {
 		size = i_size_read(mapping->host) >> huge_page_shift(h);
 		if (idx >= size)
 			goto out;
@@ -6071,6 +6071,8 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 		vma_end_reservation(h, vma, haddr);
 
 		pagecache_folio = filemap_lock_folio(mapping, idx);
+		if (IS_ERR(pagecache_folio))
+			pagecache_folio = NULL;
 	}
 
 	ptl = huge_pte_lock(h, mm, ptep);
@@ -6182,7 +6184,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	if (is_continue) {
 		ret = -EFAULT;
 		folio = filemap_lock_folio(mapping, idx);
-		if (!folio)
+		if (IS_ERR(folio))
 			goto out;
 		folio_in_pagecache = true;
 	} else if (!*pagep) {

mm/memcontrol.c

@@ -5705,7 +5705,7 @@ static struct page *mc_handle_file_pte(struct vm_area_struct *vma,
 	/* shmem/tmpfs may report page out on swap: account for that too. */
 	index = linear_page_index(vma, addr);
 	folio = filemap_get_incore_folio(vma->vm_file->f_mapping, index);
-	if (!folio)
+	if (IS_ERR(folio))
 		return NULL;
 	return folio_file_page(folio, index);
 }

mm/mincore.c

@@ -61,7 +61,7 @@ static unsigned char mincore_page(struct address_space *mapping, pgoff_t index)
 	 * tmpfs's .fault). So swapped out tmpfs mappings are tested here.
 	 */
 	folio = filemap_get_incore_folio(mapping, index);
-	if (folio) {
+	if (!IS_ERR(folio)) {
 		present = folio_test_uptodate(folio);
 		folio_put(folio);
 	}

mm/shmem.c

@@ -605,7 +605,7 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
 
 		index = (inode->i_size & HPAGE_PMD_MASK) >> PAGE_SHIFT;
 		folio = filemap_get_folio(inode->i_mapping, index);
-		if (!folio)
+		if (IS_ERR(folio))
 			goto drop;
 
 		/* No huge page at the end of the file: nothing to split */
@@ -3214,7 +3214,7 @@ static const char *shmem_get_link(struct dentry *dentry,
 
 	if (!dentry) {
 		folio = filemap_get_folio(inode->i_mapping, 0);
-		if (!folio)
+		if (IS_ERR(folio))
 			return ERR_PTR(-ECHILD);
 		if (PageHWPoison(folio_page(folio, 0)) ||
 		    !folio_test_uptodate(folio)) {