From: John Groves <[email protected]>
The comment in dax_folio_reset_order() claims that DAX maintains an
invariant where folio->share != 0 only when folio->mapping == NULL,
implying folio->share is zero whenever mapping is non-NULL. This is
misleading because folio->share and folio->index are a union -- for
non-shared folios with mapping != NULL, reading folio->share returns
the file page offset (folio->index), which is typically non-zero.
Reword the comment to accurately describe the union aliasing: the
assignment clears whichever interpretation of the union word is active
(index for non-shared folios, share for shared folios), which is correct
because the folio is being released in either case.
No functional change -- the code was already correct, only the
justification was wrong.
Fixes: 59eb73b98ae0b ("dax: Factor out dax_folio_reset_order() helper")
Signed-off-by: John Groves <[email protected]>
---
fs/dax.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
diff --git a/fs/dax.c b/fs/dax.c
index 6d175cd47a99b..df19c9317d10e 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -392,12 +392,12 @@ int dax_folio_reset_order(struct folio *folio)
int order = folio_order(folio);
/*
- * DAX maintains the invariant that folio->share != 0 only when
- * folio->mapping == NULL (enforced by dax_folio_make_shared()).
- * Equivalently: folio->mapping != NULL implies folio->share == 0.
- * Callers ensure share has been decremented to zero before
- * calling here, so unconditionally clearing both fields is
- * correct.
+ * Clear the mapping and the index/share union word. folio->share
+ * and folio->index occupy the same union in struct folio. For
+ * non-shared folios (mapping != NULL), the union holds folio->index
+ * (file page offset); for shared folios (mapping == NULL), it holds
+ * folio->share (reference count). Either way, we are releasing the
+ * folio and both fields should be zeroed.
*/
folio->mapping = NULL;
folio->share = 0;
--
2.53.0
