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>

1 files changed, 8 insertions, 6 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index ac161b50f5bc..a34abfe8c654 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1907,7 +1907,7 @@ out:
  *
  * 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 @@ repeat:
 		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 @@ no_page:
 
 		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 @@ no_page:
 			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 @@ retry_find:
 		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);