buffer: allow a buffer_head to point at memory outside the page cache

jbd2 builds a temporary buffer_head to write out the frozen copy of a
metadata block, and that copy lives in slab memory.  Today jbd2 points the
temporary buffer at the slab folio backing it.  A slab folio's ->mapping is
not an address_space, so anything that follows bh->b_folio->mapping there
gets garbage rather than NULL; mark_buffer_write_io_error() does exactly
that, and we are about to start calling it on this buffer.

Rather than teach every such helper about slab folios, allow bh->b_folio to
be NULL and let b_data point straight at the memory.  Code that needs the
folio has to check.  There are two places in this file:

 - __bh_submit() adds the data by virtual address using
   bio_add_virt_nofail(), and skips the cgroup accounting: a buffer that is
   not in the page cache has no owning folio to attribute writeback to.

 - buffer_set_crypto_ctx() returns early.  fscrypt has no interest in a
   buffer that is not part of a file mapping, which is why it already
   returns when the folio has no mapping.

Nothing sets b_folio to NULL yet, so this patch is a no-op on its own.

A buffer_head without a folio is a narrow thing, not a new general
capability.  Most of the buffer_head API assumes a folio and will fault or
corrupt state without one - touch_buffer(), bh_offset(), the async read
completion path, and plenty more - so it is up to whoever builds such a
buffer to keep it away from all of that.  What NULL buys us is that getting
it wrong fails loudly instead of quietly following a slab folio's
overloaded ->mapping.  It is also only valid over memory that is always
mapped: buffers over highmem have no permanent kernel virtual address,
which is why folio_set_bh() records a folio and an offset instead.

Suggested-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Weidong Zhu <weizhu@fiu.edu>
Signed-off-by: Chao Shi <coshi036@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/bb6fab2111a48d7ba61887fc1372362576fce6e6.1785951556.git.coshi036@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions