From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 49E2F3B8D48 for ; Tue, 11 Aug 2026 15:57:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786463854; cv=none; b=lL9G5nTd7J/6rk8RYZ4jZhEunwHe8Wut1dEtP27/2nWFJ1PPQ8Yk2TYzXOWNsGwtEhuKZf0V6wLxOgnAVOKpOEHprGQ6/Mu0KDrmugyCrLqfmj3yYG0cpJuTv8HDUvwvqLsqws9SQOwjZqWoGGaXYtlBYl+wpKVqhI1RJvsvWjk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786463854; c=relaxed/simple; bh=HPD402Ikt6O4X+8Qbqe8Zzb8py0SiP+dmsM4dd1IPtc=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=q0jXbxq35ALs92d1zuqIXx6s6gwG+5/MvjadRtZYXHWaeOaCZL6ngEkGcKeeAv/fVGOH2KN2EpZv/RXQzyWom/hKYOX1jyVj+8hJjdG7qau+VlDw+ZyDo1ffptCiieIqyNLA6Oxvm65D5B8oYMqbZ3U9qr1haWHIw+LXC3LIDFI= 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=bcvOBfGw; arc=none smtp.client-ip=170.10.129.124 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="bcvOBfGw" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1786463851; 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=bEED7qEroeMR9htSCXxZ1mOUIYUQP/wt9HQZo+ZmSXg=; b=bcvOBfGwrdP695fMpSN8dQXD7hKx3bYB0biM9cfw0RiO9rsGpxkWU1E5hAGnrfXSwYu1sn oLQoExWM+REWLJEMaEoTkSHOmGgDgIjWkhClYUmRLhv+0iYyEWCAJCw796AQbKWEprdvHf Ysz7DLCMZZRkTmLYyUQk8X7aN7YnCKI= Received: from mx-prod-mc-03.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-413-jgIb1dKqMNuz1HO8x3zkVQ-1; Tue, 11 Aug 2026 11:57:26 -0400 X-MC-Unique: jgIb1dKqMNuz1HO8x3zkVQ-1 X-Mimecast-MFC-AGG-ID: jgIb1dKqMNuz1HO8x3zkVQ_1786463843 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-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 5F63E1956045; Tue, 11 Aug 2026 15:57:23 +0000 (UTC) Received: from localhost (headnet03.pony-001.prod.iad2.dc.redhat.com [10.2.32.114]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id ADCB9195DF91; Tue, 11 Aug 2026 15:57:22 +0000 (UTC) From: Giuseppe Scrivano To: linux-erofs@lists.ozlabs.org Cc: xiang@kernel.org, linux-fsdevel@vger.kernel.org, amir73il@gmail.com, brauner@kernel.org Subject: Re: [PATCH v5 2/2] erofs: reuse superblock for file-backed mounts In-Reply-To: (Gao Xiang's message of "Tue, 11 Aug 2026 23:31:46 +0800") References: <20260811071241.389589-1-gscrivan@redhat.com> <20260811071241.389589-3-gscrivan@redhat.com> Date: Tue, 11 Aug 2026 17:57:20 +0200 Message-ID: <87bjb8wsyn.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 Gao Xiang writes: > Hi Giuseppe, > > On Tue, Aug 11, 2026 at 09:10:51AM +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 > > Thanks for the patch. > > It generally looks good to me. Just some minor nits: > >> --- >> Documentation/filesystems/erofs.rst | 8 ++++ >> fs/erofs/internal.h | 1 + >> fs/erofs/super.c | 58 +++++++++++++++++++++++++++-- >> 3 files changed, 64 insertions(+), 3 deletions(-) >> >> diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst >> index d301d9ac946a..49c4e7dbc5d6 100644 >> --- a/Documentation/filesystems/erofs.rst >> +++ b/Documentation/filesystems/erofs.rst >> @@ -139,6 +139,9 @@ inode_share Enable inode page sharing for this filesystem. Inodes wi >> page cache. >> source=%s (For file-backed mounts) Specify the backing image as a path >> or as an already-opened file descriptor. >> +superblock_share (For file-backed mounts) Share the superblock when the same >> + backing file is mounted more than once with compatible >> + options. Remount is not allowed on shared superblocks. >> =================== ========================================================= >> >> File-backed mounts >> @@ -156,6 +159,11 @@ Only regular files are accepted as backing files; to mount an image that >> resides on a block device, use the traditional block device mount path >> instead. >> >> +When superblock_share is specified and the same backing file (identified >> +by inode and fsoffset) is mounted more than once with compatible mount >> +options, the kernel reuses the existing superblock instead of creating a > > When superblock_share is specified, multiple mounts of the same backing > file at the same fsoffset with compatible mount options can share a single > superblock. Remount is now allowed on shared superblocks. > >> +new one. Remount is not allowed on shared superblocks. >> + >> Sysfs Entries >> ============= >> >> diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h >> index 580f8d9f14e7..8d65cac35702 100644 >> --- a/fs/erofs/internal.h >> +++ b/fs/erofs/internal.h >> @@ -156,6 +156,7 @@ struct erofs_sb_info { >> #define EROFS_MOUNT_DAX_NEVER 0x00000080 >> #define EROFS_MOUNT_DIRECT_IO 0x00000100 >> #define EROFS_MOUNT_INODE_SHARE 0x00000200 >> +#define EROFS_MOUNT_SUPERBLOCK_SHARE 0x00000400 >> >> #define clear_opt(opt, option) ((opt)->mount_opt &= ~EROFS_MOUNT_##option) >> #define set_opt(opt, option) ((opt)->mount_opt |= EROFS_MOUNT_##option) >> diff --git a/fs/erofs/super.c b/fs/erofs/super.c >> index d40961248a49..72d654ce49b7 100644 >> --- a/fs/erofs/super.c >> +++ b/fs/erofs/super.c >> @@ -386,7 +386,7 @@ static void erofs_default_options(struct erofs_sb_info *sbi) >> enum { >> Opt_user_xattr, Opt_acl, Opt_cache_strategy, Opt_dax, Opt_dax_enum, >> Opt_device, Opt_domain_id, Opt_directio, Opt_fsoffset, Opt_inode_share, >> - Opt_source, >> + Opt_source, Opt_superblock_share, >> }; >> >> static const struct constant_table erofs_param_cache_strategy[] = { >> @@ -415,6 +415,7 @@ static const struct fs_parameter_spec erofs_fs_parameters[] = { >> fsparam_u64("fsoffset", Opt_fsoffset), >> fsparam_flag("inode_share", Opt_inode_share), >> fsparam_file_or_string("source", Opt_source), >> + fsparam_flag("superblock_share", Opt_superblock_share), >> {} >> }; >> >> @@ -560,6 +561,12 @@ static int erofs_fc_parse_param(struct fs_context *fc, >> else >> set_opt(&sbi->opt, INODE_SHARE); >> break; >> + case Opt_superblock_share: >> + if (!IS_ENABLED(CONFIG_EROFS_FS_BACKED_BY_FILE)) >> + errorfc(fc, "%s option not supported", erofs_fs_parameters[opt].name); >> + else >> + set_opt(&sbi->opt, SUPERBLOCK_SHARE); >> + break; >> case Opt_source: >> return erofs_fc_parse_source(fc, param); >> } >> @@ -663,6 +670,17 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc) >> errorfc(fc, "FSDAX is not allowed when inode_share is on"); >> return -EINVAL; >> } >> + if (test_opt(&sbi->opt, SUPERBLOCK_SHARE) && >> + test_opt(&sbi->opt, INODE_SHARE)) { >> + errorfc(fc, "superblock_share is not allowed when inode_share is on"); >> + return -EINVAL; >> + } >> + /* Extra devices are not yet supported with superblock sharing. */ > > Okay, if the comment is added here, I don't think it's helpful since the > error message indicates the same. > >> + if (test_opt(&sbi->opt, SUPERBLOCK_SHARE) && >> + sbi->devs->extra_devices) { >> + errorfc(fc, "superblock_share does not support extra devices"); >> + return -EINVAL; >> + } >> >> sbi->blkszbits = PAGE_SHIFT; >> if (!sb->s_bdev) { >> @@ -788,6 +806,36 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc) >> return 0; >> } >> > > Maybe just move the comment below here: > > /* > * The function is used only with a sbi that has SUPERBLOCK_SHARE set, > * so that the memcmp later ensures new_sbi is also shareable. > */ > >> +static int erofs_fc_test_file_super(struct super_block *sb, > > Can we just rename it as erofs_sb_share_test_super()? > >> + struct fs_context *fc) >> +{ >> + /* >> + * The assumption is that the function is used only with a sbi that >> + * has SUPERBLOCK_SHARE set, so that the memcmp later ensures >> + * new_sbi also is shareable. >> + */ >> + struct erofs_sb_info *sbi = EROFS_SB(sb); >> + struct erofs_sb_info *new_sbi = fc->s_fs_info; >> + >> + if (sb->s_iflags & SB_I_RETIRED) >> + return 0; >> + if (!sbi->dif0.file || !new_sbi->dif0.file) >> + return 0; >> + return file_inode(sbi->dif0.file) == file_inode(new_sbi->dif0.file) && >> + sbi->dif0.fsoff == new_sbi->dif0.fsoff && >> + !memcmp(&sbi->opt, &new_sbi->opt, sizeof(sbi->opt)); >> +} >> + >> +static int erofs_fc_get_tree_file(struct fs_context *fc) > > Could we just rename it as erofs_file_get_tree()? sure, thanks for the review. I'll send another version later today/tomorrow if there are no other comments. Regards, Giuseppe