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.

[[email protected]: fix null-pointer deref]
  Link: https://lkml.kernel.org/r/[email protected]
  Link: https://lkml.kernel.org/r/20230310043137.GA1624890@u2004
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Christoph Hellwig <[email protected]>
Acked-by: Ryusuke Konishi <[email protected]> [nilfs2]
Cc: Andreas Gruenbacher <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 2 insertions, 2 deletions

diff --git a/fs/nfs/file.c b/fs/nfs/file.c
index 893625eacab9..1d03406e6c03 100644
--- a/fs/nfs/file.c
+++ b/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);