erofs: reuse superblock for file-backed mounts

When the same file-backed image is mounted multiple times (via path or
fd) with the superblock_share mount option, 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 or extra devices are used, 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=${1:-/root/fedora.erofs}
  OPTS=${2:+-o $2}
  N=20
  DIR=$(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 $OPTS "$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"

without superblock_share:

  # time ./test.sh /root/fedora.erofs
  Superblocks: 20
  erofs_inode delta: +45864
  Slab delta: +37448 kB
  real	0m2.311s
  user	0m0.222s
  sys	0m1.998s

with superblock_share:

  # time ./test.sh /root/fedora.erofs superblock_share
  Superblocks: 1
  erofs_inode delta: +2044
  Slab delta: +284 kB
  real	0m0.679s
  user	0m0.157s
  sys	0m0.469s

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 <giuseppe@scrivano.org>
Reviewed-by: Gao Xiang <xiang@kernel.org>
Signed-off-by: Gao Xiang <xiang@kernel.org>

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