[PATCH v2 04/15] 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 with 64 bit inode numbers can implement
this callback to return suitable writeback context for the inode.
In fetch_bdi_writeback_ctx(), if the callback is implemented, it is used
to obtain the context. Otherwise the default fallback computes the
writeback context by taking mod with the 32-bit inode 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          | 12 ++++++++++++
 include/linux/backing-dev.h |  4 +++-
 include/linux/fs.h          |  1 +
 4 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 5165b7cc86e2..e065cd606b80 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 0bc4b5489078..645a426d1e41 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 +1295,16 @@ 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);
+
+	return bdi->wb_ctx[ip->i_ino % 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 +1321,7 @@ 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..6f744aadd83e 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -157,7 +157,9 @@ 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[inode->i_ino % bdi->nr_wb_ctx];
 }
 
 #ifdef CONFIG_CGROUP_WRITEBACK
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8e2a041ce430..edcbc5042427 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2367,6 +2367,7 @@ struct super_operations {
 	long (*free_cached_objects)(struct super_block *,
 				    struct shrink_control *);
 	void (*shutdown)(struct super_block *sb);
+	struct bdi_writeback_ctx *(*get_inode_wb_ctx)(struct inode *inode);
 };
 
 /*
-- 
2.25.1