When the same file is mounted multiple times (via path or fd), reuse
the existing superblock instead of creating a new one.  This allows
multiple mounts of the same image to share in-kernel data structures
more efficiently.

The backing file is identified by its inode and fsoffset.  If mount
options conflict, a separate superblock is created transparently as
a fallback.

Tested by mounting a 177M Fedora EROFS image 20 times with full
traversal:

```
\#!/bin/sh
IMG=${1:-/root/fedora.erofs}
N=20
DIR=$(mktemp -d)
trap "umount $DIR/m* 2>/dev/null; rm -rf $DIR" EXIT

sync; echo 3 > /proc/sys/vm/drop_caches
INODES_BEFORE=$(grep erofs_inode /proc/slabinfo | awk '{print $2}')
MEM_BEFORE=$(grep ^Slab: /proc/meminfo | awk '{print $2}')

for i in $(seq 1 $N); do mkdir $DIR/m$i && mount -t erofs "$IMG" $DIR/m$i && find $DIR/m$i > /dev/null; done

echo "Superblocks: $(grep $DIR /proc/self/mountinfo | awk '{print $3}' | sort -u | wc -l)"
echo "erofs_inode delta: +$(( $(grep erofs_inode /proc/slabinfo | awk '{print $2}') - INODES_BEFORE ))"
echo "Slab delta: +$(( $(grep ^Slab: /proc/meminfo | awk '{print $2}') - MEM_BEFORE )) kB"
```

unpatched kernel:

Superblocks: 20
erofs_inode delta: +46368
Slab delta: +39952 kB
0.08user 0.82system 0:00.98elapsed 93%CPU (0avgtext+0avgdata 4796maxresident)k
19168inputs+10304outputs (24major+16672minor)pagefaults 0swaps

patched kernel:

Superblocks: 1
erofs_inode delta: +2044
Slab delta: +1088 kB
0.07user 0.21system 0:00.31elapsed 91%CPU (0avgtext+0avgdata 4848maxresident)k
19024inputs+6784outputs (25major+16810minor)pagefaults 0swaps

The time difference shows that sharing the superblock also benefits
the page cache and inode cache, as subsequent mounts of the same image
avoid re-reading the backing file.  This is particularly useful for
container hosts running multiple containers from the same base image.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
---
 Documentation/filesystems/erofs.rst |  4 +++
 fs/erofs/super.c                    | 44 +++++++++++++++++++++++++++--
 2 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst
index 774e8b236d09..ef6785035b26 100644
--- a/Documentation/filesystems/erofs.rst
+++ b/Documentation/filesystems/erofs.rst
@@ -154,6 +154,10 @@ Only regular files are accepted as backing files; to mount an image that
 resides on a block device, use the traditional block device mount path
 instead.
 
+When the same backing file is mounted more than once with compatible
+mount options, the kernel reuses the existing superblock.  If mount
+options conflict, a separate superblock is created transparently.
+
 Sysfs Entries
 =============
 
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 558041011398..cbd76727f0d3 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -788,6 +788,46 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 	return 0;
 }
 
+static int erofs_fc_test_file_super(struct super_block *sb,
+				    struct fs_context *fc)
+{
+	struct erofs_sb_info *sbi = EROFS_SB(sb);
+	struct erofs_sb_info *new_sbi = fc->s_fs_info;
+
+	if (sb->s_iflags & SB_I_RETIRED)
+		return 0;
+	if (!sbi->dif0.file || !new_sbi->dif0.file)
+		return 0;
+	return file_inode(sbi->dif0.file) == file_inode(new_sbi->dif0.file) &&
+	       sbi->dif0.fsoff == new_sbi->dif0.fsoff &&
+	       sbi->opt.mount_opt == new_sbi->opt.mount_opt &&
+	       sbi->opt.cache_strategy == new_sbi->opt.cache_strategy;
+}
+
+static int erofs_fc_get_tree_file(struct fs_context *fc)
+{
+	struct super_block *sb;
+	int err;
+
+	sb = sget_fc(fc, erofs_fc_test_file_super, set_anon_super_fc);
+	if (IS_ERR(sb))
+		return PTR_ERR(sb);
+
+	if (!sb->s_root) {
+		err = erofs_fc_fill_super(sb, fc);
+		if (err) {
+			deactivate_locked_super(sb);
+			return err;
+		}
+		sb->s_flags |= SB_ACTIVE;
+	} else {
+		erofs_info(sb, "sharing superblock for the same backing file");
+	}
+
+	fc->root = dget(sb->s_root);
+	return 0;
+}
+
 static int erofs_fc_get_tree(struct fs_context *fc)
 {
 	struct erofs_sb_info *sbi = fc->s_fs_info;
@@ -803,7 +843,7 @@ static int erofs_fc_get_tree(struct fs_context *fc)
 			errorfc(fc, "source is unsupported");
 			return -EINVAL;
 		}
-		return get_tree_nodev(fc, erofs_fc_fill_super);
+		return erofs_fc_get_tree_file(fc);
 	}
 
 	ret = get_tree_bdev_flags(fc, erofs_fc_fill_super,
@@ -821,7 +861,7 @@ static int erofs_fc_get_tree(struct fs_context *fc)
 
 		if (S_ISREG(file_inode(sbi->dif0.file)->i_mode) &&
 		    sbi->dif0.file->f_mapping->a_ops->read_folio)
-			return get_tree_nodev(fc, erofs_fc_fill_super);
+			return erofs_fc_get_tree_file(fc);
 	}
 	return ret;
 }
-- 
2.55.0