From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BBDE7C34026 for ; Tue, 18 Feb 2020 09:22:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 91A5C20658 for ; Tue, 18 Feb 2020 09:22:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726312AbgBRJWg (ORCPT ); Tue, 18 Feb 2020 04:22:36 -0500 Received: from mx2.suse.de ([195.135.220.15]:36438 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726293AbgBRJWg (ORCPT ); Tue, 18 Feb 2020 04:22:36 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id B448FACCE; Tue, 18 Feb 2020 09:22:32 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id BAE0D1E0CF7; Tue, 18 Feb 2020 10:22:31 +0100 (CET) Date: Tue, 18 Feb 2020 10:22:31 +0100 From: Jan Kara To: Sascha Hauer Cc: Al Viro , Jan Kara , linux-fsdevel@vger.kernel.org, linux-mtd@lists.infradead.org, Jan Kara , Richard Weinberger , kernel@pengutronix.de Subject: Re: [PATCH 1/8] quota: Allow to pass mount path to quotactl Message-ID: <20200218092231.GG16121@quack2.suse.cz> References: <20200124131323.23885-1-s.hauer@pengutronix.de> <20200124131323.23885-2-s.hauer@pengutronix.de> <20200127104518.GC19414@quack2.suse.cz> <20200128100631.zv7cn726twylcmb7@pengutronix.de> <20200129012929.GV23230@ZenIV.linux.org.uk> <20200204103523.tbxzptf4lkr474yi@pengutronix.de> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="gKMricLos+KVdGMg" Content-Disposition: inline In-Reply-To: <20200204103523.tbxzptf4lkr474yi@pengutronix.de> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org --gKMricLos+KVdGMg Content-Type: text/plain; charset=us-ascii Content-Disposition: inline I'm sorry for the late reply, I was busy with other things and I wasn't quite sure how I'd like this to be handled :) On Tue 04-02-20 11:35:23, Sascha Hauer wrote: > On Wed, Jan 29, 2020 at 01:29:29AM +0000, Al Viro wrote: > > On Tue, Jan 28, 2020 at 11:06:31AM +0100, Sascha Hauer wrote: > > > Hi Jan, > > > > > @@ -810,6 +811,36 @@ static struct super_block *quotactl_block(const char __user *special, int cmd) > > > #endif > > > } > > > > > > +static struct super_block *quotactl_path(const char __user *special, int cmd, > > > + struct path *path) > > > +{ > > > + struct super_block *sb; > > > + int ret; > > > + > > > + ret = user_path_at(AT_FDCWD, special, LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT, > > > + path); > > > + if (ret) > > > + return ERR_PTR(ret); > > > + > > > + sb = path->mnt->mnt_sb; > > > +restart: > > > + if (quotactl_cmd_onoff(cmd)) > > > + down_write(&sb->s_umount); > > > + else > > > + down_read(&sb->s_umount); > > > + > > > + if (quotactl_cmd_write(cmd) && sb->s_writers.frozen != SB_UNFROZEN) { > > > + if (quotactl_cmd_onoff(cmd)) > > > + up_write(&sb->s_umount); > > > + else > > > + up_read(&sb->s_umount); > > > + wait_event(sb->s_writers.wait_unfrozen, > > > + sb->s_writers.frozen == SB_UNFROZEN); > > > + goto restart; > > > + } > > > + > > > + return sb; > > > +} > > > > This partial duplicate of __get_super_thawed() guts does *not* belong here, > > especially not interleaved with quota-specific checks. > > > > > + if (q_path) { > > > + if (quotactl_cmd_onoff(cmd)) > > > + up_write(&sb->s_umount); > > > + else > > > + up_read(&sb->s_umount); > > > + > > > + path_put(&sb_path); > > > + } else { > > > + if (!quotactl_cmd_onoff(cmds)) > > > + drop_super(sb); > > > + else > > > + drop_super_exclusive(sb); > > > + } > > > > Er... Why not have the same code that you've used to lock the damn thing > > (needs to be moved to fs/super.c) simply get a passive ref to it? Then > > you could do the same thing, q_path or no q_path... > > I am getting confused here. To an earlier version of this series you > responded: > > > And for path-based you don't need to mess with superblock > > references - just keep the struct path until the end. That > > will keep the superblock alive and active just fine. > > I did that and got the objection from Jan: > > > So I've realized that just looking up superblock with user_path_at() is not > > enough. Quota code also expects that the superblock will be locked > > (sb->s_umount) and filesystem will not be frozen (in case the quota > > operation is going to modify the filesystem). This is needed to serialize > > e.g. remount and quota operations or quota operations among themselves. Yes, using passive reference is not necessary. On the other hand the symmetry with how get_super() and friends work has some appeal too so if Al wants that, well, he's the maintainer ;) > So after drawing circles we now seem to be back at passive references. > What I have now in my tree is this in fs/super.c, untested currently: I was thinking how to make the API most sensible. In the end I've decided for a variant that is attached - we pass in struct path which enforces active reference to a superblock and thus we don't have to be afraid of the superblock going away or similar problems. Also the operation "get me superblock for a path" kind of makes sense... Guys, what do you think? Honza -- Jan Kara SUSE Labs, CR --gKMricLos+KVdGMg Content-Type: text/x-patch; charset=us-ascii Content-Disposition: attachment; filename="0001-fs-Provide-functions-for-getting-locked-and-thawed-s.patch" >From 18c7913475342f10b4723c7e22409acc573e6436 Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Tue, 18 Feb 2020 10:05:21 +0100 Subject: [PATCH] fs: Provide functions for getting locked and thawed superblock from a path Provide functions to get locked (with s_umount) and if desired also thawed superblock for a given struct path. We additionally also get passive reference count of the superblock so that the superblock can be unlocked with put_super() / put_super_exlusive() similarly to how get_super_*() class of functions operates. Signed-off-by: Jan Kara --- fs/super.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ include/linux/fs.h | 3 +++ 2 files changed, 67 insertions(+) diff --git a/fs/super.c b/fs/super.c index cd352530eca9..2c78eac57fa5 100644 --- a/fs/super.c +++ b/fs/super.c @@ -836,6 +836,70 @@ struct super_block *get_super_exclusive_thawed(struct block_device *bdev) } EXPORT_SYMBOL(get_super_exclusive_thawed); +static struct super_block *__hold_super(struct path *path, bool excl, + bool thawed) +{ + struct super_block *sb = path->mnt->mnt_sb; + + while (1) { + if (excl) + down_write(&sb->s_umount); + else + down_read(&sb->s_umount); + if (!thawed || sb->s_writers.frozen == SB_UNFROZEN) { + spin_lock(&sb_lock); + sb->s_count++; + spin_unlock(&sb_lock); + return sb; + } + if (excl) + up_write(&sb->s_umount); + else + up_read(&sb->s_umount); + wait_event(sb->s_writers.wait_unfrozen, + sb->s_writers.frozen == SB_UNFROZEN); + } +} + +/** + * hold_super - get locked superblock for a path + * @path: path to get superblock for + * + * This function gets superblock for @path and returns it with s_umount + * held in shared mode and superblock's passive refcount (sb->s_count) + * incremented. + */ +struct super_block *hold_super(struct path *path) +{ + return __hold_super(path, false, false); +} + +/** + * hold_super_thawed - get locked thawed superblock for a path + * @path: path to get superblock for + * + * This function gets superblock for @path, makes sure it is not frozen, + * and returns it with s_umount held in shared mode and superblock's + * passive refcount (sb->s_count) incremented. + */ +struct super_block *hold_super_thawed(struct path *path) +{ + return __hold_super(path, false, true); +} + +/** + * hold_super_thawed_exclusive - get locked thawed superblock for a path + * @path: path to get superblock for + * + * This function gets superblock for @path, makes sure it is not frozen, + * and returns it with s_umount held in exclusive mode and superblock's + * passive refcount (sb->s_count) incremented. + */ +struct super_block *hold_super_thawed_exclusive(struct path *path) +{ + return __hold_super(path, true, true); +} + /** * get_active_super - get an active reference to the superblock of a device * @bdev: device to get the superblock for diff --git a/include/linux/fs.h b/include/linux/fs.h index 3cd4fe6b845e..2406494d2f54 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -3305,6 +3305,9 @@ extern struct file_system_type *get_fs_type(const char *name); extern struct super_block *get_super(struct block_device *); extern struct super_block *get_super_thawed(struct block_device *); extern struct super_block *get_super_exclusive_thawed(struct block_device *bdev); +struct super_block *hold_super(struct path *path); +struct super_block *hold_super_thawed(struct path *path); +struct super_block *hold_super_thawed_exclusive(struct path *path); extern struct super_block *get_active_super(struct block_device *bdev); extern void drop_super(struct super_block *sb); extern void drop_super_exclusive(struct super_block *sb); -- 2.16.4 --gKMricLos+KVdGMg--