doc: improve the description of __folio_mark_dirty

Patch series "Improve buffer head documentation", v3.

Turn buffer head documentation into its own document, and make many
general improvements to the docs.  Obviously there is much more that could
be done.  Tested with make htmldocs.


This patch (of 8):

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.

Link: https://lkml.kernel.org/r/20240416031754.4076917-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20240416031754.4076917-2-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page-writeback.c

@@ -2705,11 +2705,15 @@ 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 (e.g.
+ * 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)