[RFC PATCH 1/2] erofs: iomap support for non-tailpacking DIO

[Gao Xiang <hsiangkao@linux.alibaba.com>]

From: Huang Jianan <huangjianan@oppo.com>

Add iomap support for non-tailpacking uncompressed data in order to
support DIO and DAX.

Direct I/O is useful in certain scenarios for uncompressed files.
For example, double pagecache can be avoid by direct I/O when
loop device is used for uncompressed files containing upper layer
compressed filesystem.

This adds iomap DIO support for non-tailpacking cases first and
tail-packing inline files can be handled later.

Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Huang Jianan <huangjianan@oppo.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
---
 fs/erofs/Kconfig    |   1 +
 fs/erofs/data.c     | 102 ++++++++++++++++++++++++++++++++++++++++++++
 fs/erofs/inode.c    |   5 ++-
 fs/erofs/internal.h |   1 +
 4 files changed, 108 insertions(+), 1 deletion(-)

diff --git a/fs/erofs/Kconfig b/fs/erofs/Kconfig
index 906af0c1998c..14b747026742 100644
--- a/fs/erofs/Kconfig
+++ b/fs/erofs/Kconfig
@@ -3,6 +3,7 @@
 config EROFS_FS
 	tristate "EROFS filesystem support"
 	depends on BLOCK
+	select FS_IOMAP
 	select LIBCRC32C
 	help
 	  EROFS (Enhanced Read-Only File System) is a lightweight
diff --git a/fs/erofs/data.c b/fs/erofs/data.c
index 3787a5fb0a42..38e9439c2510 100644
--- a/fs/erofs/data.c
+++ b/fs/erofs/data.c
@@ -5,6 +5,7 @@
  */
 #include "internal.h"
 #include <linux/prefetch.h>
+#include <linux/iomap.h>
 
 #include <trace/events/erofs.h>
 
@@ -308,9 +309,110 @@ static sector_t erofs_bmap(struct address_space *mapping, sector_t block)
 	return 0;
 }
 
+static int erofs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
+		unsigned int flags, struct iomap *iomap, struct iomap *srcmap)
+{
+	int ret;
+	struct erofs_map_blocks map;
+
+	map.m_la = offset;
+	map.m_llen = length;
+
+	ret = erofs_map_blocks_flatmode(inode, &map, EROFS_GET_BLOCKS_RAW);
+	if (ret < 0)
+		return ret;
+
+	iomap->bdev = inode->i_sb->s_bdev;
+	iomap->offset = map.m_la;
+	iomap->length = map.m_llen;
+
+	if (!(map.m_flags & EROFS_MAP_MAPPED)) {
+		iomap->type = IOMAP_HOLE;
+		iomap->addr = IOMAP_NULL_ADDR;
+		if (!iomap->length)
+			iomap->length = length;
+		return 0;
+	}
+
+	/* that shouldn't happen for now */
+	if (map.m_flags & EROFS_MAP_META) {
+		DBG_BUGON(1);
+		return -ENOTBLK;
+	}
+	iomap->type = IOMAP_MAPPED;
+	iomap->addr = map.m_pa;
+	iomap->flags = 0;
+	return 0;
+}
+
+const struct iomap_ops erofs_iomap_ops = {
+	.iomap_begin = erofs_iomap_begin,
+};
+
+static int erofs_prepare_dio(struct kiocb *iocb, struct iov_iter *to)
+{
+	struct inode *inode = file_inode(iocb->ki_filp);
+	unsigned int blksize_mask = (1 << inode->i_blkbits) - 1;
+	loff_t align = iocb->ki_pos | iov_iter_count(to) |
+		iov_iter_alignment(to);
+	struct block_device *bdev = inode->i_sb->s_bdev;
+
+	if (bdev)
+		blksize_mask = (1 << ilog2(bdev_logical_block_size(bdev))) - 1;
+	else
+		blksize_mask = (1 << inode->i_blkbits) - 1;
+
+	if (align & blksize_mask)
+		return -EINVAL;
+
+	/*
+	 * Tail-packing inline data is not supported for iomap for now.
+	 * Temporarily fall back this to buffered I/O instead.
+	 */
+	if (EROFS_I(inode)->datalayout == EROFS_INODE_FLAT_INLINE &&
+	    iocb->ki_pos + iov_iter_count(to) >
+			rounddown(inode->i_size, EROFS_BLKSIZ))
+		return 1;
+	return 0;
+}
+
+static ssize_t erofs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
+{
+	/* no need taking (shared) inode lock since it's a ro filesystem */
+	if (!iov_iter_count(to))
+		return 0;
+
+	if (iocb->ki_flags & IOCB_DIRECT) {
+		int err = erofs_prepare_dio(iocb, to);
+
+		if (!err)
+			return iomap_dio_rw(iocb, to, &erofs_iomap_ops,
+					    NULL, 0);
+		if (err < 0)
+			return err;
+		/*
+		 * Fallback to buffered I/O if the operation being performed on
+		 * the inode is not supported by direct I/O. The IOCB_DIRECT
+		 * flag needs to be cleared here in order to ensure that the
+		 * direct I/O path within generic_file_read_iter() is not
+		 * taken.
+		 */
+		iocb->ki_flags &= ~IOCB_DIRECT;
+	}
+	return generic_file_read_iter(iocb, to);
+}
+
 /* for uncompressed (aligned) files and raw access for other files */
 const struct address_space_operations erofs_raw_access_aops = {
 	.readpage = erofs_raw_access_readpage,
 	.readahead = erofs_raw_access_readahead,
 	.bmap = erofs_bmap,
+	.direct_IO = noop_direct_IO,
+};
+
+const struct file_operations erofs_file_fops = {
+	.llseek		= generic_file_llseek,
+	.read_iter	= erofs_file_read_iter,
+	.mmap		= generic_file_readonly_mmap,
+	.splice_read	= generic_file_splice_read,
 };
diff --git a/fs/erofs/inode.c b/fs/erofs/inode.c
index aa8a0d770ba3..00edb7562fea 100644
--- a/fs/erofs/inode.c
+++ b/fs/erofs/inode.c
@@ -247,7 +247,10 @@ static int erofs_fill_inode(struct inode *inode, int isdir)
 	switch (inode->i_mode & S_IFMT) {
 	case S_IFREG:
 		inode->i_op = &erofs_generic_iops;
-		inode->i_fop = &generic_ro_fops;
+		if (!erofs_inode_is_data_compressed(vi->datalayout))
+			inode->i_fop = &erofs_file_fops;
+		else
+			inode->i_fop = &generic_ro_fops;
 		break;
 	case S_IFDIR:
 		inode->i_op = &erofs_dir_iops;
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index 543c2ff97d30..2669c785d548 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -371,6 +371,7 @@ static inline int z_erofs_map_blocks_iter(struct inode *inode,
 #endif	/* !CONFIG_EROFS_FS_ZIP */
 
 /* data.c */
+extern const struct file_operations erofs_file_fops;
 struct page *erofs_get_meta_page(struct super_block *sb, erofs_blk_t blkaddr);
 
 /* inode.c */
-- 
2.24.4