[PATCH v2 04/16] writeback: affine inode to a writeback ctx within a bdi

[Kundan Kumar <kundan.kumar@samsung.com>]

Affine inode to a writeback context. This helps in minimizing the
filesytem fragmentation due to inode being processed by different
threads.

To support parallel writeback, wire up a new superblock operation
get_inode_wb_ctx(). Filesystems can override this callback and select
desired writeback context for a inode. FS can use the wb context based
on its geometry and also use 64 bit inode numbers.

If a filesystem doesn't implement this callback, it defaults to
DEFALT_WB_CTX = 0, maintaining its original behavior.

An example implementation for XFS is provided, where XFS selects the
writeback context based on its Allocation Group number.

Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
Signed-off-by: Kundan Kumar <kundan.kumar@samsung.com>
---
 fs/fs-writeback.c           |  3 ++-
 fs/xfs/xfs_super.c          | 13 +++++++++++++
 include/linux/backing-dev.h |  5 ++++-
 include/linux/fs.h          |  1 +
 4 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 0715a7617391..56c048e22f72 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -265,7 +265,8 @@ void __inode_attach_wb(struct inode *inode, struct folio *folio)
 {
 	struct backing_dev_info *bdi = inode_to_bdi(inode);
 	struct bdi_writeback *wb = NULL;
-	struct bdi_writeback_ctx *bdi_writeback_ctx = bdi->wb_ctx[0];
+	struct bdi_writeback_ctx *bdi_writeback_ctx =
+						fetch_bdi_writeback_ctx(inode);
 
 	if (inode_cgwb_enabled(inode)) {
 		struct cgroup_subsys_state *memcg_css;
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index bb0a82635a77..b3ec9141d902 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -53,6 +53,7 @@
 #include <linux/magic.h>
 #include <linux/fs_context.h>
 #include <linux/fs_parser.h>
+#include <linux/backing-dev.h>
 
 static const struct super_operations xfs_super_operations;
 
@@ -1294,6 +1295,17 @@ xfs_fs_show_stats(
 	return 0;
 }
 
+static struct bdi_writeback_ctx *
+xfs_get_inode_wb_ctx(
+	struct inode		*inode)
+{
+	struct xfs_inode *ip = XFS_I(inode);
+	struct backing_dev_info *bdi = inode_to_bdi(inode);
+	xfs_agino_t agno = XFS_INO_TO_AGNO(ip->i_mount, ip->i_ino);
+
+	return bdi->wb_ctx[agno % bdi->nr_wb_ctx];
+}
+
 static const struct super_operations xfs_super_operations = {
 	.alloc_inode		= xfs_fs_alloc_inode,
 	.destroy_inode		= xfs_fs_destroy_inode,
@@ -1310,6 +1322,7 @@ static const struct super_operations xfs_super_operations = {
 	.free_cached_objects	= xfs_fs_free_cached_objects,
 	.shutdown		= xfs_fs_shutdown,
 	.show_stats		= xfs_fs_show_stats,
+	.get_inode_wb_ctx       = xfs_get_inode_wb_ctx,
 };
 
 static int
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 951ab5497500..59bbb69d300c 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -148,6 +148,7 @@ static inline bool mapping_can_writeback(struct address_space *mapping)
 	return inode_to_bdi(mapping->host)->capabilities & BDI_CAP_WRITEBACK;
 }
 
+#define DEFAULT_WB_CTX 0
 #define for_each_bdi_wb_ctx(bdi, wbctx) \
 	for (int __i = 0; __i < (bdi)->nr_wb_ctx \
 		&& ((wbctx) = (bdi)->wb_ctx[__i]) != NULL; __i++)
@@ -157,7 +158,9 @@ fetch_bdi_writeback_ctx(struct inode *inode)
 {
 	struct backing_dev_info *bdi = inode_to_bdi(inode);
 
-	return bdi->wb_ctx[0];
+	if (inode->i_sb->s_op->get_inode_wb_ctx)
+		return inode->i_sb->s_op->get_inode_wb_ctx(inode);
+	return bdi->wb_ctx[DEFAULT_WB_CTX];
 }
 
 #ifdef CONFIG_CGROUP_WRITEBACK
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 754fec84f350..5199b0d49fa5 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2379,6 +2379,7 @@ struct super_operations {
 	 */
 	int (*remove_bdev)(struct super_block *sb, struct block_device *bdev);
 	void (*shutdown)(struct super_block *sb);
+	struct bdi_writeback_ctx *(*get_inode_wb_ctx)(struct inode *inode);
 };
 
 /*
-- 
2.25.1