[PATCH 1/5] doc: Improve the description of __folio_mark_dirty

[Matthew Wilcox (Oracle)]

I've learned why it's safe to call __folio_mark_dirty() from
mark_buffer_dirty() without holding the folio lock, so update
the description to explain why.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 mm/page-writeback.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index cd4e4ae77c40..96da6716cb86 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2652,11 +2652,15 @@ void folio_account_cleaned(struct folio *folio, struct bdi_writeback *wb)
  * If warn is true, then emit a warning if the folio is not uptodate and has
  * not been truncated.
  *
- * The caller must hold folio_memcg_lock().  Most callers have the folio
- * locked.  A few have the folio blocked from truncation through other
- * means (eg zap_vma_pages() has it mapped and is holding the page table
- * lock).  This can also be called from mark_buffer_dirty(), which I
- * cannot prove is always protected against truncate.
+ * The caller must hold folio_memcg_lock().  It is the caller's
+ * responsibility to prevent the folio from being truncated while
+ * this function is in progress, although it may have been truncated
+ * before this function is called.  Most callers have the folio locked.
+ * A few have the folio blocked from truncation through other means (eg
+ * zap_vma_pages() has it mapped and is holding the page table lock).
+ * When called from mark_buffer_dirty(), the filesystem should hold a
+ * reference to the buffer_head that is being marked dirty, which causes
+ * try_to_free_buffers() to fail.
  */
 void __folio_mark_dirty(struct folio *folio, struct address_space *mapping,
 			     int warn)
-- 
2.43.0