From: John Groves 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") Reviewed-by: Jonathan Cameron Signed-off-by: John Groves --- 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