[PATCH 3/8] block: enable RWF_DONTCACHE for block devices

Christoph Hellwig
2026-04-09T16:03:23Z

From: Tal Zussman <tz2294@columbia.edu>

Block device buffered reads and writes already pass through
filemap_read() and iomap_file_buffered_write() respectively, both of
which handle IOCB_DONTCACHE. Enable RWF_DONTCACHE for block device files
by setting FOP_DONTCACHE in def_blk_fops.

For CONFIG_BUFFER_HEAD=y paths, add block_write_begin_iocb() which
threads the kiocb through so that buffer_head-based I/O can use
DONTCACHE behavior. 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 so that buffer_head
writeback completions get deferred to task context.

CONFIG_BUFFER_HEAD=n paths are handled by the previously added iomap
BIO_COMPLETE_IN_TASK support.

This support is useful for databases that operate on raw block devices,
among other userspace applications.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
block/fops.c | 5 +++--
fs/buffer.c | 22 +++++++++++++++++++---
include/linux/buffer_head.h | 3 +++
3 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/block/fops.c b/block/fops.c
index bb6642b45937..31b073181d87 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -504,7 +504,8 @@ static int blkdev_write_begin(const struct kiocb *iocb,
unsigned len, struct folio **foliop,
void **fsdata)
{
- return block_write_begin(mapping, pos, len, foliop, blkdev_get_block);
+ return block_write_begin_iocb(iocb, mapping, pos, len, foliop,
+ blkdev_get_block);
}

static int blkdev_write_end(const struct kiocb *iocb,
@@ -966,7 +967,7 @@ const struct file_operations def_blk_fops = {
.splice_write = iter_file_splice_write,
.fallocate = blkdev_fallocate,
.uring_cmd = blkdev_uring_cmd,
- .fop_flags = FOP_BUFFER_RASYNC,
+ .fop_flags = FOP_BUFFER_RASYNC | FOP_DONTCACHE,
};

static __init int blkdev_init(void)
diff --git a/fs/buffer.c b/fs/buffer.c
index d6e062c42a8d..289ab33fe3fd 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2131,14 +2131,19 @@ 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);
@@ -2153,6 +2158,13 @@ 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,
@@ -2481,7 +2493,8 @@ int cont_write_begin(const struct kiocb *iocb, struct address_space *mapping,
(*bytes)++;
}

- return block_write_begin(mapping, pos, len, foliop, get_block);
+ return block_write_begin_iocb(iocb, mapping, pos, len, foliop,
+ get_block);
}
EXPORT_SYMBOL(cont_write_begin);

@@ -2711,6 +2724,9 @@ static void submit_bh_wbc(blk_opf_t opf, struct buffer_head *bh,
if (IS_ENABLED(CONFIG_FS_ENCRYPTION))
buffer_set_crypto_ctx(bio, bh, GFP_NOIO);

+ if (folio_test_dropbehind(bh->b_folio))
+ bio_set_flag(bio, BIO_COMPLETE_IN_TASK);
+
bio->bi_iter.bi_sector = bh->b_blocknr * (bh->b_size >> 9);
bio->bi_write_hint = write_hint;

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 *);
--
2.47.3