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 <[email protected]>
---
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
