[PATCH v3 2/3] ntfs: avoid heap allocation for free-cluster readahead state

[DaeMyung Kang]

get_nr_free_clusters() is run from the precalc_free_clusters() worker
queued at the end of ntfs_fill_super().  It is also the only place that
publishes the result by atomic64_set(&vol->free_clusters, ...), sets
NVolSetFreeClusterKnown(), and wakes vol->free_waitq.

The function currently allocates a temporary file_ra_state with
kzalloc() before that publication happens.  If the allocation fails,
get_nr_free_clusters() returns 0 without setting NVolFreeClusterKnown()
or waking vol->free_waitq, so callers that wait for the free count can
block forever.

The readahead state is only used synchronously while scanning the bitmap
and struct file_ra_state is small.  Keep it on the stack and pass it to
the readahead helper by address, eliminating the early allocation
failure path.  Zero-initialize the on-stack state because
file_ra_state_init() only sets ra_pages and prev_pos.

Signed-off-by: DaeMyung Kang <charsyam@xxxxxxxxx>
---
 fs/ntfs/super.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/fs/ntfs/super.c b/fs/ntfs/super.c
index 7e3561265b47..d2c4fb6ea0d4 100644
--- a/fs/ntfs/super.c
+++ b/fs/ntfs/super.c
@@ -1954,7 +1954,7 @@ s64 get_nr_free_clusters(struct ntfs_volume *vol)
 	struct address_space *mapping = vol->lcnbmp_ino->i_mapping;
 	struct folio *folio;
 	pgoff_t index, max_index;
-	struct file_ra_state *ra;
+	struct file_ra_state ra = { 0 };
 
 	ntfs_debug("Entering.");
 	/* Serialize accesses to the cluster bitmap. */
@@ -1962,11 +1962,7 @@ s64 get_nr_free_clusters(struct ntfs_volume *vol)
 	if (NVolFreeClusterKnown(vol))
 		return atomic64_read(&vol->free_clusters);
 
-	ra = kzalloc(sizeof(*ra), GFP_NOFS);
-	if (!ra)
-		return 0;
-
-	file_ra_state_init(ra, mapping);
+	file_ra_state_init(&ra, mapping);
 
 	/*
 	 * Convert the number of bits into bytes rounded up, then convert into
@@ -1985,7 +1981,7 @@ s64 get_nr_free_clusters(struct ntfs_volume *vol)
 		 * Get folio from page cache, getting it from backing store
 		 * if necessary, and increment the use count.
 		 */
-		folio = ntfs_get_locked_folio(mapping, index, max_index, ra);
+		folio = ntfs_get_locked_folio(mapping, index, max_index, &ra);
 
 		/* Ignore pages which errored synchronously. */
 		if (IS_ERR(folio)) {
@@ -2024,7 +2020,6 @@ s64 get_nr_free_clusters(struct ntfs_volume *vol)
 	else
 		atomic64_set(&vol->free_clusters, nr_free);
 
-	kfree(ra);
 	NVolSetFreeClusterKnown(vol);
 	wake_up_all(&vol->free_waitq);
 	ntfs_debug("Exiting.");
-- 
2.43.0