Re: [PATCH v6 2/2] erofs: reuse superblock for file-backed mounts

On Wed, Aug 12, 2026 at 04:28:04PM +0200, Giuseppe Scrivano wrote:
> 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 <[email protected]>

Reviewed-by: Gao Xiang <[email protected]>

Thanks,
Gao Xiang