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

Pedro Falcato <[email protected]> writes:

> 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?)

This is the same preexisting behavior for block device mounts.  That
said, I realize this can feel confusing since EROFS is not a block
device and allowed this so far.

One way to solve this could be an explicit mount option "share_sb" that
is opt-in and, once set, blocks any remount operations.  Would that
work?

Regards,
Giuseppe