From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E479339DBD7 for ; Thu, 30 Jul 2026 13:49:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785419365; cv=none; b=sNLIRPBRU2lcbGRPAKTwcTxP+GaZL8eTqQi2VbEavwjMhUIT13kGrvBwoYNHKfm5S+6VpssxIHaax4cukiUPrl7PHvU1xcyNcbXyIXNf2zHLqDRMU+xnJXXNNLL+YYf32tgc4i8iK/SK1TWqEGXmiA0uUZTURnbyIDFlZRuEyx4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785419365; c=relaxed/simple; bh=8qwv5PHUKAWevhYZKKooQP58IC17+6pzYZvIcyxMNQs=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=rBSmXX7EUu9WW94ZPz1ezh3arj5XJnrUPlPeLivj1jHPz20NNdYLlZP8tJoOg1ufi5adQGZW4Hx0CntMIiNVkkyn93C0Y3EI1ateWEL1JAKMG5RuGK4B5XmhdJV9PkFpuSsdNvL3AwTYLN5LbW3vEsqFrmqt8sfQWTJcW6jazXM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=ICnlsd1A; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="ICnlsd1A" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1785419362; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=b4EqHffQZ+7yOTPccLs7D+jOVNUfFcke1CghDWdjZE8=; b=ICnlsd1ANoYDPAYXLOzZwj633o3Cr6KUI9w/1rJiY1W3TLPAvwcJDIMraUeKKbCQ8c15m5 UZrMapZqQ1+EcJw77Qpwsb+baHUxfdvySyPhtjAXpysYhPGZekiA5IXtHnXrDxiL8hqxl7 kuq9/Vaxvqc61mcQ5WFYo5CaHLFsDpA= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-638-shRJlmpBPsm3LdaYnEvU1Q-1; Thu, 30 Jul 2026 09:49:19 -0400 X-MC-Unique: shRJlmpBPsm3LdaYnEvU1Q-1 X-Mimecast-MFC-AGG-ID: shRJlmpBPsm3LdaYnEvU1Q_1785419357 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E1561195608D; Thu, 30 Jul 2026 13:49:16 +0000 (UTC) Received: from localhost (unknown [10.44.49.125]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 390E11956089; Thu, 30 Jul 2026 13:49:15 +0000 (UTC) From: Giuseppe Scrivano To: Pedro Falcato Cc: linux-erofs@lists.ozlabs.org, xiang@kernel.org, linux-fsdevel@vger.kernel.org, amir73il@gmail.com Subject: Re: [PATCH] erofs: reuse superblock for file-backed mounts In-Reply-To: (Pedro Falcato's message of "Thu, 30 Jul 2026 13:57:29 +0100") References: <20260730124120.1501126-1-gscrivan@redhat.com> Date: Thu, 30 Jul 2026 15:49:14 +0200 Message-ID: <8733x0k2p1.fsf@redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Pedro Falcato 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