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=-16.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 2C98EC10DC2 for ; Tue, 19 Jan 2021 14:57:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 00C8620DD4 for ; Tue, 19 Jan 2021 14:57:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2391641AbhASOys (ORCPT ); Tue, 19 Jan 2021 09:54:48 -0500 Received: from mail.kernel.org ([198.145.29.99]:55848 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387793AbhASLuU (ORCPT ); Tue, 19 Jan 2021 06:50:20 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 9A07022DD3; Tue, 19 Jan 2021 11:49:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1611056979; bh=ZgIU27w3FETjaR2QKFzazWTre2YNu51/x6Mln/h4RYs=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=TUfiw3V26qVobQilVW6oH4ys4uRRvsZrsA/zjiZkKQcYMN/4fX+aCAxZaOcsBTRrK YZYzlM4wxBb0/X70svX9ZEh1ctfN549ZcbbJ4NJI//hOwB5WJJ7NqHT6uDZXyYZYTO FrygnMRHAIv2v0xLAv691FT1tFicGXdXbGs11qqixw7yk3c4ZYNeJrvyobCmPf1PYU ocmudjWc/HrqWszSszGvlnIExLNwsF8KDPvVKSpHX0DLNuZZa1xXfcNjJ/SAzqRu2j yHde8KXmKtV3OAynuRrpMH/aq5tOfrWNP0XeJuGGvxRdBomk4fOjILEV/duwp3uoY+ 42pQjhwmBV3Yg== Message-ID: <9820d213cefd20f90227ed5a73a55fa41d4df6da.camel@kernel.org> Subject: Re: [PATCH v4] overlay: Implement volatile-specific fsync error behaviour From: Jeff Layton To: Sargun Dhillon , linux-unionfs@vger.kernel.org, miklos@szeredi.hu, Amir Goldstein Cc: Alexander Viro , Giuseppe Scrivano , Vivek Goyal , Daniel J Walsh , linux-fsdevel@vger.kernel.org, David Howells , Chengguang Xu , Christoph Hellwig , NeilBrown , Jan Kara , stable@vger.kernel.org, Matthew Wilcox Date: Tue, 19 Jan 2021 06:49:36 -0500 In-Reply-To: <20210108001043.12683-1-sargun@sargun.me> References: <20210108001043.12683-1-sargun@sargun.me> Content-Type: text/plain; charset="ISO-8859-15" User-Agent: Evolution 3.38.3 (3.38.3-1.fc33) MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-unionfs@vger.kernel.org On Thu, 2021-01-07 at 16:10 -0800, Sargun Dhillon wrote: > Overlayfs's volatile option allows the user to bypass all forced sync calls > to the upperdir filesystem. This comes at the cost of safety. We can never > ensure that the user's data is intact, but we can make a best effort to > expose whether or not the data is likely to be in a bad state. > > The best way to handle this in the time being is that if an overlayfs's > upperdir experiences an error after a volatile mount occurs, that error > will be returned on fsync, fdatasync, sync, and syncfs. This is > contradictory to the traditional behaviour of VFS which fails the call > once, and only raises an error if a subsequent fsync error has occurred, > and been raised by the filesystem. > > One awkward aspect of the patch is that we have to manually set the > superblock's errseq_t after the sync_fs callback as opposed to just > returning an error from syncfs. This is because the call chain looks > something like this: > > sys_syncfs -> > sync_filesystem -> > __sync_filesystem -> > /* The return value is ignored here > sb->s_op->sync_fs(sb) > _sync_blockdev > /* Where the VFS fetches the error to raise to userspace */ > errseq_check_and_advance > > Because of this we call errseq_set every time the sync_fs callback occurs. > Due to the nature of this seen / unseen dichotomy, if the upperdir is an > inconsistent state at the initial mount time, overlayfs will refuse to > mount, as overlayfs cannot get a snapshot of the upperdir's errseq that > will increment on error until the user calls syncfs. > > Signed-off-by: Sargun Dhillon > Suggested-by: Amir Goldstein > Reviewed-by: Amir Goldstein > Fixes: c86243b090bc ("ovl: provide a mount option "volatile"") > Cc: linux-fsdevel@vger.kernel.org > Cc: linux-unionfs@vger.kernel.org > Cc: stable@vger.kernel.org > Cc: Jeff Layton > Cc: Miklos Szeredi > Cc: Amir Goldstein > Cc: Vivek Goyal > Cc: Matthew Wilcox > --- >  Documentation/filesystems/overlayfs.rst | 8 ++++++ >  fs/overlayfs/file.c | 5 ++-- >  fs/overlayfs/overlayfs.h | 1 + >  fs/overlayfs/ovl_entry.h | 2 ++ >  fs/overlayfs/readdir.c | 5 ++-- >  fs/overlayfs/super.c | 34 ++++++++++++++++++++----- >  fs/overlayfs/util.c | 27 ++++++++++++++++++++ >  7 files changed, 71 insertions(+), 11 deletions(-) > > diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst > index 580ab9a0fe31..137afeb3f581 100644 > --- a/Documentation/filesystems/overlayfs.rst > +++ b/Documentation/filesystems/overlayfs.rst > @@ -575,6 +575,14 @@ without significant effort. >  The advantage of mounting with the "volatile" option is that all forms of >  sync calls to the upper filesystem are omitted. >   > > > > +In order to avoid a giving a false sense of safety, the syncfs (and fsync) > +semantics of volatile mounts are slightly different than that of the rest of > +VFS. If any writeback error occurs on the upperdir's filesystem after a > +volatile mount takes place, all sync functions will return an error. Once this > +condition is reached, the filesystem will not recover, and every subsequent sync > +call will return an error, even if the upperdir has not experience a new error > +since the last sync call. > + >  When overlay is mounted with "volatile" option, the directory >  "$workdir/work/incompat/volatile" is created. During next mount, overlay >  checks for this directory and refuses to mount if present. This is a strong > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c > index a1f72ac053e5..5c5c3972ebd0 100644 > --- a/fs/overlayfs/file.c > +++ b/fs/overlayfs/file.c > @@ -445,8 +445,9 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync) >   const struct cred *old_cred; >   int ret; >   > > > > - if (!ovl_should_sync(OVL_FS(file_inode(file)->i_sb))) > - return 0; > + ret = ovl_sync_status(OVL_FS(file_inode(file)->i_sb)); > + if (ret <= 0) > + return ret; >   > > > >   ret = ovl_real_fdget_meta(file, &real, !datasync); >   if (ret) > diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h > index f8880aa2ba0e..9f7af98ae200 100644 > --- a/fs/overlayfs/overlayfs.h > +++ b/fs/overlayfs/overlayfs.h > @@ -322,6 +322,7 @@ int ovl_check_metacopy_xattr(struct ovl_fs *ofs, struct dentry *dentry); >  bool ovl_is_metacopy_dentry(struct dentry *dentry); >  char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry, >   int padding); > +int ovl_sync_status(struct ovl_fs *ofs); >   > > > >  static inline bool ovl_is_impuredir(struct super_block *sb, >   struct dentry *dentry) > diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h > index 1b5a2094df8e..b208eba5d0b6 100644 > --- a/fs/overlayfs/ovl_entry.h > +++ b/fs/overlayfs/ovl_entry.h > @@ -79,6 +79,8 @@ struct ovl_fs { >   atomic_long_t last_ino; >   /* Whiteout dentry cache */ >   struct dentry *whiteout; > + /* r/o snapshot of upperdir sb's only taken on volatile mounts */ > + errseq_t errseq; >  }; >   > > > >  static inline struct vfsmount *ovl_upper_mnt(struct ovl_fs *ofs) > diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c > index 01620ebae1bd..a273ef901e57 100644 > --- a/fs/overlayfs/readdir.c > +++ b/fs/overlayfs/readdir.c > @@ -909,8 +909,9 @@ static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end, >   struct file *realfile; >   int err; >   > > > > - if (!ovl_should_sync(OVL_FS(file->f_path.dentry->d_sb))) > - return 0; > + err = ovl_sync_status(OVL_FS(file->f_path.dentry->d_sb)); > + if (err <= 0) > + return err; >   > > > >   realfile = ovl_dir_real_file(file, true); >   err = PTR_ERR_OR_ZERO(realfile); > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c > index 290983bcfbb3..d23177a53c95 100644 > --- a/fs/overlayfs/super.c > +++ b/fs/overlayfs/super.c > @@ -261,11 +261,20 @@ static int ovl_sync_fs(struct super_block *sb, int wait) >   struct super_block *upper_sb; >   int ret; >   > > > > - if (!ovl_upper_mnt(ofs)) > - return 0; > + ret = ovl_sync_status(ofs); > + /* > + * We have to always set the err, because the return value isn't > + * checked in syncfs, and instead indirectly return an error via > + * the sb's writeback errseq, which VFS inspects after this call. > + */ > + if (ret < 0) { > + errseq_set(&sb->s_wb_err, -EIO); > + return -EIO; > + } > + Apologies for the late review: You're converting the error you got back from errseq_check to -EIO unconditionally here. Why? Would it not be better to just pass the error through? Like this: if (ret < 0) { errseq_set(&sb->s_wb_err, ret); return ret; } ? > + if (!ret) > + return ret; >   > > > > - if (!ovl_should_sync(ofs)) > - return 0; >   /* >   * Not called for sync(2) call or an emergency sync (SB_I_SKIP_SYNC). >   * All the super blocks will be iterated, including upper_sb. > @@ -1927,6 +1936,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) >   sb->s_op = &ovl_super_operations; >   > > > >   if (ofs->config.upperdir) { > + struct super_block *upper_sb; > + >   if (!ofs->config.workdir) { >   pr_err("missing 'workdir'\n"); >   goto out_err; > @@ -1936,6 +1947,16 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) >   if (err) >   goto out_err; >   > > > > + upper_sb = ovl_upper_mnt(ofs)->mnt_sb; > + if (!ovl_should_sync(ofs)) { > + ofs->errseq = errseq_sample(&upper_sb->s_wb_err); > + if (errseq_check(&upper_sb->s_wb_err, ofs->errseq)) { > + err = -EIO; > + pr_err("Cannot mount volatile when upperdir has an unseen error. Sync upperdir fs to clear state.\n"); > + goto out_err; > + } > + } > + >   err = ovl_get_workdir(sb, ofs, &upperpath); >   if (err) >   goto out_err; > @@ -1943,9 +1964,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) >   if (!ofs->workdir) >   sb->s_flags |= SB_RDONLY; >   > > > > - sb->s_stack_depth = ovl_upper_mnt(ofs)->mnt_sb->s_stack_depth; > - sb->s_time_gran = ovl_upper_mnt(ofs)->mnt_sb->s_time_gran; > - > + sb->s_stack_depth = upper_sb->s_stack_depth; > + sb->s_time_gran = upper_sb->s_time_gran; >   } >   oe = ovl_get_lowerstack(sb, splitlower, numlower, ofs, layers); >   err = PTR_ERR(oe); > diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c > index 23f475627d07..6e7b8c882045 100644 > --- a/fs/overlayfs/util.c > +++ b/fs/overlayfs/util.c > @@ -950,3 +950,30 @@ char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry, >   kfree(buf); >   return ERR_PTR(res); >  } > + > +/* > + * ovl_sync_status() - Check fs sync status for volatile mounts > + * > + * Returns 1 if this is not a volatile mount and a real sync is required. > + * > + * Returns 0 if syncing can be skipped because mount is volatile, and no errors > + * have occurred on the upperdir since the mount. > + * > + * Returns -errno if it is a volatile mount, and the error that occurred since > + * the last mount. If the error code changes, it'll return the latest error > + * code. > + */ > + > +int ovl_sync_status(struct ovl_fs *ofs) > +{ > + struct vfsmount *mnt; > + > + if (ovl_should_sync(ofs)) > + return 1; > + > + mnt = ovl_upper_mnt(ofs); > + if (!mnt) > + return 0; > + > + return errseq_check(&mnt->mnt_sb->s_wb_err, ofs->errseq); > +} Aside from my minor question about converting the error unconditionally, this all looks fine. You can add: Reviewed-by: Jeff Layton Nice work!