[PATCH 1/2] iomap: make buffered writes work with RWF_DONTCACHE

[Jens Axboe <axboe@kernel.dk>]

Add iomap buffered write support for RWF_DONTCACHE. If RWF_DONTCACHE is
set for a write, mark the folios being written as uncached. Then
writeback completion will drop the pages. The write_iter handler simply
kicks off writeback for the pages, and writeback completion will take
care of the rest.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 Documentation/filesystems/iomap/design.rst     | 5 +++++
 Documentation/filesystems/iomap/operations.rst | 2 ++
 fs/iomap/buffered-io.c                         | 4 ++++
 include/linux/iomap.h                          | 1 +
 4 files changed, 12 insertions(+)

diff --git a/Documentation/filesystems/iomap/design.rst b/Documentation/filesystems/iomap/design.rst
index b0d0188a095e..7b91546750f5 100644
--- a/Documentation/filesystems/iomap/design.rst
+++ b/Documentation/filesystems/iomap/design.rst
@@ -352,6 +352,11 @@ operations:
    ``IOMAP_NOWAIT`` is often set on behalf of ``IOCB_NOWAIT`` or
    ``RWF_NOWAIT``.
 
+ * ``IOMAP_DONTCACHE`` is set when the caller wishes to perform a
+   buffered file I/O and would like the kernel to drop the pagecache
+   after the I/O completes, if it isn't already being used by another
+   thread.
+
 If it is necessary to read existing file contents from a `different
 <https://lore.kernel.org/all/20191008071527.29304-9-hch@lst.de/>`_
 device or address range on a device, the filesystem should return that
diff --git a/Documentation/filesystems/iomap/operations.rst b/Documentation/filesystems/iomap/operations.rst
index 2c7f5df9d8b0..584ff549f9a6 100644
--- a/Documentation/filesystems/iomap/operations.rst
+++ b/Documentation/filesystems/iomap/operations.rst
@@ -131,6 +131,8 @@ These ``struct kiocb`` flags are significant for buffered I/O with iomap:
 
  * ``IOCB_NOWAIT``: Turns on ``IOMAP_NOWAIT``.
 
+ * ``IOCB_DONTCACHE``: Turns on ``IOMAP_DONTCACHE``.
+
 Internal per-Folio State
 ------------------------
 
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index d303e6c8900c..ea863c3cf510 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -603,6 +603,8 @@ struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos, size_t len)
 
 	if (iter->flags & IOMAP_NOWAIT)
 		fgp |= FGP_NOWAIT;
+	if (iter->flags & IOMAP_DONTCACHE)
+		fgp |= FGP_DONTCACHE;
 	fgp |= fgf_set_order(len);
 
 	return __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
@@ -1034,6 +1036,8 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
 
 	if (iocb->ki_flags & IOCB_NOWAIT)
 		iter.flags |= IOMAP_NOWAIT;
+	if (iocb->ki_flags & IOCB_DONTCACHE)
+		iter.flags |= IOMAP_DONTCACHE;
 
 	while ((ret = iomap_iter(&iter, ops)) > 0)
 		iter.processed = iomap_write_iter(&iter, i);
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 75bf54e76f3b..26b0dbe23e62 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -183,6 +183,7 @@ struct iomap_folio_ops {
 #define IOMAP_DAX		0
 #endif /* CONFIG_FS_DAX */
 #define IOMAP_ATOMIC		(1 << 9)
+#define IOMAP_DONTCACHE		(1 << 10)
 
 struct iomap_ops {
 	/*
-- 
2.47.2