When the same file-backed image is mounted multiple times (via path or
fd) with the superblock_share mount option and matching domain_id, 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.
Superblock sharing requires both superblock_share and domain_id to be
specified, following the same pattern as inode_share. The backing file
is identified by its inode and fsoffset. If mount options conflict, a
separate superblock is created transparently as a fallback. Remount is
not allowed on shared superblocks.
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 -o domain_id=test,superblock_share "$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
patched kernel:
Superblocks: 1
erofs_inode delta: +2044
Slab delta: +1088 kB
0.07user 0.21system 0:00.31elapsed
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]>
---
v1:
https://lore.kernel.org/linux-fsdevel/[email protected]/
Needs:
https://lore.kernel.org/linux-fsdevel/[email protected]/
Documentation/filesystems/erofs.rst | 16 ++++++-
fs/erofs/internal.h | 1 +
fs/erofs/super.c | 72 +++++++++++++++++++++++++++--
3 files changed, 83 insertions(+), 6 deletions(-)
diff --git a/Documentation/filesystems/erofs.rst
b/Documentation/filesystems/erofs.rst
index 774e8b236d09..517c526163e2 100644
--- a/Documentation/filesystems/erofs.rst
+++ b/Documentation/filesystems/erofs.rst
@@ -131,12 +131,17 @@ device=%s Specify a path to an extra device
to be used together.
directio (For file-backed mounts) Use direct I/O to access
backing
files, and asynchronous I/O will be enabled if
supported.
domain_id=%s Specify a trusted domain ID. Filesystems sharing the
same
- domain ID can share page cache across mounts when inode
- page sharing is enabled. (not shown in mountinfo output)
+ domain ID can share page cache across mounts when
+ inode_share is enabled, and share superblocks when
+ superblock_share is enabled. (not shown in mountinfo
+ output)
fsoffset=%llu Specify block-aligned filesystem offset for the primary
device.
inode_share Enable inode page sharing for this filesystem. Inodes
with
identical content within the same domain ID can share
the
page cache.
+superblock_share Share the superblock when the same backing file is
mounted
+ more than once with the same domain_id and compatible
+ options. Remount is not allowed on shared superblocks.
===================
=========================================================
File-backed mounts
@@ -154,6 +159,13 @@ 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 domain_id and superblock_share are both specified and the same
+backing file (identified by inode and fsoffset) is mounted more than
+once with the same domain_id and compatible mount options, the kernel
+reuses the existing superblock instead of creating a new one.
+
+Remount is not allowed on shared superblocks.
+
Sysfs Entries
=============
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index 580f8d9f14e7..8d65cac35702 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -156,6 +156,7 @@ struct erofs_sb_info {
#define EROFS_MOUNT_DAX_NEVER 0x00000080
#define EROFS_MOUNT_DIRECT_IO 0x00000100
#define EROFS_MOUNT_INODE_SHARE 0x00000200
+#define EROFS_MOUNT_SUPERBLOCK_SHARE 0x00000400
#define clear_opt(opt, option) ((opt)->mount_opt &= ~EROFS_MOUNT_##option)
#define set_opt(opt, option) ((opt)->mount_opt |= EROFS_MOUNT_##option)
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 558041011398..23fee43de1d1 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -386,7 +386,7 @@ static void erofs_default_options(struct erofs_sb_info *sbi)
enum {
Opt_user_xattr, Opt_acl, Opt_cache_strategy, Opt_dax, Opt_dax_enum,
Opt_device, Opt_domain_id, Opt_directio, Opt_fsoffset, Opt_inode_share,
- Opt_source,
+ Opt_source, Opt_superblock_share,
};
static const struct constant_table erofs_param_cache_strategy[] = {
@@ -415,6 +415,7 @@ static const struct fs_parameter_spec erofs_fs_parameters[]
= {
fsparam_u64("fsoffset", Opt_fsoffset),
fsparam_flag("inode_share", Opt_inode_share),
fsparam_file_or_string("source", Opt_source),
+ fsparam_flag("superblock_share", Opt_superblock_share),
{}
};
@@ -536,7 +537,8 @@ static int erofs_fc_parse_param(struct fs_context *fc,
++sbi->devs->extra_devices;
break;
case Opt_domain_id:
- if (!IS_ENABLED(CONFIG_EROFS_FS_PAGE_CACHE_SHARE)) {
+ if (!IS_ENABLED(CONFIG_EROFS_FS_PAGE_CACHE_SHARE) &&
+ !IS_ENABLED(CONFIG_EROFS_FS_BACKED_BY_FILE)) {
errorfc(fc, "%s option not supported",
erofs_fs_parameters[opt].name);
} else {
kfree_sensitive(sbi->domain_id);
@@ -560,6 +562,12 @@ static int erofs_fc_parse_param(struct fs_context *fc,
else
set_opt(&sbi->opt, INODE_SHARE);
break;
+ case Opt_superblock_share:
+ if (!IS_ENABLED(CONFIG_EROFS_FS_BACKED_BY_FILE))
+ errorfc(fc, "%s option not supported",
erofs_fs_parameters[opt].name);
+ else
+ set_opt(&sbi->opt, SUPERBLOCK_SHARE);
+ break;
case Opt_source:
return erofs_fc_parse_source(fc, param);
}
@@ -659,6 +667,10 @@ static int erofs_fc_fill_super(struct super_block *sb,
struct fs_context *fc)
errorfc(fc, "domain_id is needed when inode_ishare is on");
return -EINVAL;
}
+ if (!sbi->domain_id && test_opt(&sbi->opt, SUPERBLOCK_SHARE)) {
+ errorfc(fc, "domain_id is needed when superblock_share is on");
+ return -EINVAL;
+ }
if (test_opt(&sbi->opt, DAX_ALWAYS) && test_opt(&sbi->opt,
INODE_SHARE)) {
errorfc(fc, "FSDAX is not allowed when inode_ishare is on");
return -EINVAL;
@@ -788,6 +800,55 @@ 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;
+ if (!test_opt(&new_sbi->opt, SUPERBLOCK_SHARE))
+ return 0;
+ if (!sbi->domain_id || !new_sbi->domain_id ||
+ strcmp(sbi->domain_id, new_sbi->domain_id))
+ 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 erofs_sb_info *sbi = fc->s_fs_info;
+ struct super_block *sb;
+ int err;
+
+ if (!test_opt(&sbi->opt, SUPERBLOCK_SHARE))
+ return get_tree_nodev(fc, erofs_fc_fill_super);
+
+ 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) {
+ erofs_info(sb, "sharing superblock for the same backing file");
+ } else {
+ err = erofs_fc_fill_super(sb, fc);
+ if (err) {
+ deactivate_locked_super(sb);
+ return err;
+ }
+ sb->s_flags |= SB_ACTIVE;
+ }
+
+ 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 +864,7 @@ 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 +882,7 @@ 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;
}
@@ -834,6 +895,9 @@ static int erofs_fc_reconfigure(struct fs_context *fc)
DBG_BUGON(!sb_rdonly(sb));
+ if (sbi->domain_id && test_opt(&sbi->opt, SUPERBLOCK_SHARE))
+ return -EBUSY;
+
if (new_sbi->domain_id)
erofs_info(sb, "ignoring reconfiguration for domain_id.");
--
2.55.0
