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

On Thu, Jul 30, 2026 at 02:41:20PM +0200, Giuseppe Scrivano wrote:
> 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.

Doesn't this approach break-down as soon as you get to change SB flags (through
e.g mount -o remount)? It will change superblock flags for all of them, right?

(I don't think you can switch off the superblock transparently on a
reconfigure?)


-- 
Pedro