[PATCH v2 02/21] buffer: allow a buffer_head to point at memory outside the page cache

[Chao Shi]

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@xxxxxxxxxxxxx>
Acked-by: Weidong Zhu <weizhu@xxxxxxx>
Signed-off-by: Chao Shi <coshi036@xxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
---
 fs/buffer.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index be8b57a635cd..04fcc34e4fa6 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1099,12 +1099,16 @@ EXPORT_SYMBOL(__bforget);
 static void buffer_set_crypto_ctx(struct bio *bio, const struct buffer_head *bh,
 				  gfp_t gfp_mask)
 {
-	const struct address_space *mapping = folio_mapping(bh->b_folio);
+	const struct address_space *mapping;
 
 	/*
 	 * The ext4 journal (jbd2) can submit a buffer_head it directly created
-	 * for a non-pagecache page.  fscrypt doesn't care about these.
+	 * for memory that is not in the page cache at all.  fscrypt doesn't
+	 * care about these.
 	 */
+	if (!bh->b_folio)
+		return;
+	mapping = folio_mapping(bh->b_folio);
 	if (!mapping)
 		return;
 	fscrypt_set_bio_crypt_ctx(bio, mapping->host,
@@ -1142,7 +1146,11 @@ static void __bh_submit(struct buffer_head *bh, blk_opf_t opf,
 	bio->bi_iter.bi_sector = bh->b_blocknr * (bh->b_size >> 9);
 	bio->bi_write_hint = write_hint;
 
-	bio_add_folio_nofail(bio, bh->b_folio, bh->b_size, bh_offset(bh));
+	if (bh->b_folio)
+		bio_add_folio_nofail(bio, bh->b_folio, bh->b_size,
+				     bh_offset(bh));
+	else
+		bio_add_virt_nofail(bio, bh->b_data, bh->b_size);
 
 	bio->bi_end_io = end_bio;
 	bio->bi_private = bh;
@@ -1152,7 +1160,8 @@ static void __bh_submit(struct buffer_head *bh, blk_opf_t opf,
 
 	if (wbc) {
 		wbc_init_bio(wbc, bio);
-		wbc_account_cgroup_owner(wbc, bh->b_folio, bh->b_size);
+		if (bh->b_folio)
+			wbc_account_cgroup_owner(wbc, bh->b_folio, bh->b_size);
 	}
 
 	blk_crypto_submit_bio(bio);
-- 
2.43.0