[PATCH v6 3/4] buffer: add dropbehind writeback support

[Tal Zussman <tz2294@columbia.edu>]

Add block_write_begin_iocb() which threads the kiocb through to
__filemap_get_folio() so that buffer_head-based I/O can use DONTCACHE
behavior. When the iocb has IOCB_DONTCACHE set, FGP_DONTCACHE is
passed to mark the folio for dropbehind. The existing
block_write_begin() is preserved as a wrapper that passes a NULL iocb.

Set BIO_COMPLETE_IN_TASK in submit_bh_wbc() when the folio has
dropbehind set, so that buffer_head writeback completions get deferred
to task context.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 fs/buffer.c                 | 19 +++++++++++++++++--
 include/linux/buffer_head.h |  3 +++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index b0b3792b1496..d0abaf44d782 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2138,14 +2138,19 @@ EXPORT_SYMBOL(block_commit_write);
  *
  * The filesystem needs to handle block truncation upon failure.
  */
-int block_write_begin(struct address_space *mapping, loff_t pos, unsigned len,
+int block_write_begin_iocb(const struct kiocb *iocb,
+		struct address_space *mapping, loff_t pos, unsigned len,
 		struct folio **foliop, get_block_t *get_block)
 {
 	pgoff_t index = pos >> PAGE_SHIFT;
+	fgf_t fgp_flags = FGP_WRITEBEGIN;
 	struct folio *folio;
 	int status;
 
-	folio = __filemap_get_folio(mapping, index, FGP_WRITEBEGIN,
+	if (iocb && iocb->ki_flags & IOCB_DONTCACHE)
+		fgp_flags |= FGP_DONTCACHE;
+
+	folio = __filemap_get_folio(mapping, index, fgp_flags,
 			mapping_gfp_mask(mapping));
 	if (IS_ERR(folio))
 		return PTR_ERR(folio);
@@ -2160,6 +2165,13 @@ int block_write_begin(struct address_space *mapping, loff_t pos, unsigned len,
 	*foliop = folio;
 	return status;
 }
+
+int block_write_begin(struct address_space *mapping, loff_t pos, unsigned len,
+		struct folio **foliop, get_block_t *get_block)
+{
+	return block_write_begin_iocb(NULL, mapping, pos, len, foliop,
+				      get_block);
+}
 EXPORT_SYMBOL(block_write_begin);
 
 int block_write_end(loff_t pos, unsigned len, unsigned copied,
@@ -2715,6 +2727,9 @@ static void submit_bh_wbc(blk_opf_t opf, struct buffer_head *bh,
 
 	bio = bio_alloc(bh->b_bdev, 1, opf, GFP_NOIO);
 
+	if (folio_test_dropbehind(bh->b_folio))
+		bio_set_flag(bio, BIO_COMPLETE_IN_TASK);
+
 	if (IS_ENABLED(CONFIG_FS_ENCRYPTION))
 		buffer_set_crypto_ctx(bio, bh, GFP_NOIO);
 
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index e4939e33b4b5..4ce50882d621 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -260,6 +260,9 @@ int block_read_full_folio(struct folio *, get_block_t *);
 bool block_is_partially_uptodate(struct folio *, size_t from, size_t count);
 int block_write_begin(struct address_space *mapping, loff_t pos, unsigned len,
 		struct folio **foliop, get_block_t *get_block);
+int block_write_begin_iocb(const struct kiocb *iocb,
+		struct address_space *mapping, loff_t pos, unsigned len,
+		struct folio **foliop, get_block_t *get_block);
 int __block_write_begin(struct folio *folio, loff_t pos, unsigned len,
 		get_block_t *get_block);
 int block_write_end(loff_t pos, unsigned len, unsigned copied, struct folio *);

-- 
2.39.5