Re: [PATCH V2] ovl: fsync after metadata copy-up via mount option "fsync=strict"

From: Amir Goldstein
Date: Mon Jul 22 2024 - 09:56:55 EST

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In reply to: Fei Lv: "[PATCH V2] ovl: fsync after metadata copy-up via mount option "fsync=strict""
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On Mon, Jul 22, 2024 at 1:14 PM Fei Lv <feilv@xxxxxxxxxxxx> wrote:
>
> For upper filesystem which does not enforce ordering on storing of
> metadata changes(e.g. ubifs), when overlayfs file is modified for
> the first time, copy up will create a copy of the lower file and
> its parent directories in the upper layer. Permission lost of the
> new upper parent directory was observed during power-cut stress test.
>
> Fix by adding new mount opion "fsync=strict", make sure data/metadata of
> copied up directory written to disk before renaming from tmp to final
> destination.
>
> Signed-off-by: Fei Lv <feilv@xxxxxxxxxxxx>

Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>

but I'd also like to wait for an ACK from Miklos on this feature.

As for timing, we are in the middle of the merge window for 6.11-rc1,
so we have some time before this can be considered for 6.12.
I will be on vacation for most of this development cycle, so either
Miklos will be able to queue it for 6.12 or I may be able to do
near the end of the 6.11 cycle.

Thanks,
Amir.

> ---
> V1 -> V2:
> 1. change open flags from "O_LARGEFILE | O_WRONLY" to "O_RDONLY".
> 2. change mount option to "fsync=ordered/strict/volatile".
> 3. ovl_should_sync_strict() implies ovl_should_sync().
> 4. remove redundant ovl_should_sync_strict from ovl_copy_up_meta_inode_data.
> 5. update commit log.
> 6. update documentation overlayfs.rst.
>
> Documentation/filesystems/overlayfs.rst | 39 +++++++++++++++++++++++++
> fs/overlayfs/copy_up.c | 18 ++++++++++++
> fs/overlayfs/ovl_entry.h | 20 +++++++++++--
> fs/overlayfs/params.c | 33 ++++++++++++++++++---
> fs/overlayfs/super.c | 2 +-
> 5 files changed, 105 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
> index 165514401441..a783e57bdb57 100644
> --- a/Documentation/filesystems/overlayfs.rst
> +++ b/Documentation/filesystems/overlayfs.rst
> @@ -742,6 +742,45 @@ controlled by the "uuid" mount option, which supports these values:
> mounted with "uuid=on".
>
>
> +Durability and copy up
> +----------------------
> +
> +The fsync(2) and fdatasync(2) system calls ensure that the metadata and
> +data of a file, respectively, are safely written to the backing
> +storage, which is expected to guarantee the existence of the information post
> +system crash.
> +
> +Without the fdatasync(2) call, there is no guarantee that the observed
> +data after a system crash will be either the old or the new data, but
> +in practice, the observed data after crash is often the old or new data or a
> +mix of both.
> +
> +When overlayfs file is modified for the first time, copy up will create
> +a copy of the lower file and its parent directories in the upper layer.
> +In case of a system crash, if fdatasync(2) was not called after the
> +modification, the upper file could end up with no data at all (i.e.
> +zeros), which would be an unusual outcome. To avoid this experience,
> +overlayfs calls fsync(2) on the upper file before completing the copy up with
> +rename(2) to make the copy up "atomic".
> +
> +Depending on the backing filesystem (e.g. ubifs), fsync(2) before
> +rename(2) may not be enough to provide the "atomic" copy up behavior
> +and fsync(2) on the copied up parent directories is required as well.
> +
> +Overlayfs can be tuned to prefer performance or durability when storing
> +to the underlying upper layer. This is controlled by the "fsync" mount
> +option, which supports these values:
> +
> +- "ordered": (default)
> + Call fsync(2) on upper file before completion of copy up.
> +- "strict":
> + Call fsync(2) on upper file and directories before completion of copy up.
> +- "volatile": [*]
> + Prefer performance over durability (see `Volatile mount`_)
> +
> +[*] The mount option "volatile" is an alias to "fsync=volatile".
> +
> +
> Volatile mount
> --------------
>
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index a5ef2005a2cc..d99a18afceb8 100644
> --- a/fs/overlayfs/copy_up.c
> +++ b/fs/overlayfs/copy_up.c
> @@ -243,6 +243,21 @@ static int ovl_verify_area(loff_t pos, loff_t pos2, loff_t len, loff_t totlen)
> return 0;
> }
>
> +static int ovl_copy_up_sync(struct path *path)
> +{
> + struct file *new_file;
> + int err;
> +
> + new_file = ovl_path_open(path, O_RDONLY);
> + if (IS_ERR(new_file))
> + return PTR_ERR(new_file);
> +
> + err = vfs_fsync(new_file, 0);
> + fput(new_file);
> +
> + return err;
> +}
> +
> static int ovl_copy_up_file(struct ovl_fs *ofs, struct dentry *dentry,
> struct file *new_file, loff_t len)
> {
> @@ -701,6 +716,9 @@ static int ovl_copy_up_metadata(struct ovl_copy_up_ctx *c, struct dentry *temp)
> err = ovl_set_attr(ofs, temp, &c->stat);
> inode_unlock(temp->d_inode);
>
> + if (!err && ovl_should_sync_strict(ofs))
> + err = ovl_copy_up_sync(&upperpath);
> +
> return err;
> }
>
> diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> index cb449ab310a7..7f6d2effd5f1 100644
> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -5,6 +5,12 @@
> * Copyright (C) 2016 Red Hat, Inc.
> */
>
> +enum {
> + OVL_FSYNC_ORDERED,
> + OVL_FSYNC_STRICT,
> + OVL_FSYNC_VOLATILE,
> +};
> +
> struct ovl_config {
> char *upperdir;
> char *workdir;
> @@ -18,7 +24,7 @@ struct ovl_config {
> int xino;
> bool metacopy;
> bool userxattr;
> - bool ovl_volatile;
> + int fsync_mode;
> };
>
> struct ovl_sb {
> @@ -120,7 +126,17 @@ static inline struct ovl_fs *OVL_FS(struct super_block *sb)
>
> static inline bool ovl_should_sync(struct ovl_fs *ofs)
> {
> - return !ofs->config.ovl_volatile;
> + return ofs->config.fsync_mode != OVL_FSYNC_VOLATILE;
> +}
> +
> +static inline bool ovl_should_sync_strict(struct ovl_fs *ofs)
> +{
> + return ofs->config.fsync_mode == OVL_FSYNC_STRICT;
> +}
> +
> +static inline bool ovl_is_volatile(struct ovl_config *config)
> +{
> + return config->fsync_mode == OVL_FSYNC_VOLATILE;
> }
>
> static inline unsigned int ovl_numlower(struct ovl_entry *oe)
> diff --git a/fs/overlayfs/params.c b/fs/overlayfs/params.c
> index 4860fcc4611b..c4aac288b7e0 100644
> --- a/fs/overlayfs/params.c
> +++ b/fs/overlayfs/params.c
> @@ -58,6 +58,7 @@ enum ovl_opt {
> Opt_xino,
> Opt_metacopy,
> Opt_verity,
> + Opt_fsync,
> Opt_volatile,
> };
>
> @@ -139,6 +140,23 @@ static int ovl_verity_mode_def(void)
> return OVL_VERITY_OFF;
> }
>
> +static const struct constant_table ovl_parameter_fsync[] = {
> + { "ordered", OVL_FSYNC_ORDERED },
> + { "strict", OVL_FSYNC_STRICT },
> + { "volatile", OVL_FSYNC_VOLATILE },
> + {}
> +};
> +
> +static const char *ovl_fsync_mode(struct ovl_config *config)
> +{
> + return ovl_parameter_fsync[config->fsync_mode].name;
> +}
> +
> +static int ovl_fsync_mode_def(void)
> +{
> + return OVL_FSYNC_ORDERED;
> +}
> +
> const struct fs_parameter_spec ovl_parameter_spec[] = {
> fsparam_string_empty("lowerdir", Opt_lowerdir),
> fsparam_string("lowerdir+", Opt_lowerdir_add),
> @@ -154,6 +172,7 @@ const struct fs_parameter_spec ovl_parameter_spec[] = {
> fsparam_enum("xino", Opt_xino, ovl_parameter_xino),
> fsparam_enum("metacopy", Opt_metacopy, ovl_parameter_bool),
> fsparam_enum("verity", Opt_verity, ovl_parameter_verity),
> + fsparam_enum("fsync", Opt_fsync, ovl_parameter_fsync),
> fsparam_flag("volatile", Opt_volatile),
> {}
> };
> @@ -617,8 +636,11 @@ static int ovl_parse_param(struct fs_context *fc, struct fs_parameter *param)
> case Opt_verity:
> config->verity_mode = result.uint_32;
> break;
> + case Opt_fsync:
> + config->fsync_mode = result.uint_32;
> + break;
> case Opt_volatile:
> - config->ovl_volatile = true;
> + config->fsync_mode = OVL_FSYNC_VOLATILE;
> break;
> case Opt_userxattr:
> config->userxattr = true;
> @@ -802,9 +824,9 @@ int ovl_fs_params_verify(const struct ovl_fs_context *ctx,
> config->index = false;
> }
>
> - if (!config->upperdir && config->ovl_volatile) {
> + if (!config->upperdir && ovl_is_volatile(config)) {
> pr_info("option \"volatile\" is meaningless in a non-upper mount, ignoring it.\n");
> - config->ovl_volatile = false;
> + config->fsync_mode = ovl_fsync_mode_def();
> }
>
> if (!config->upperdir && config->uuid == OVL_UUID_ON) {
> @@ -997,8 +1019,11 @@ int ovl_show_options(struct seq_file *m, struct dentry *dentry)
> if (ofs->config.metacopy != ovl_metacopy_def)
> seq_printf(m, ",metacopy=%s",
> ofs->config.metacopy ? "on" : "off");
> - if (ofs->config.ovl_volatile)
> + if (ovl_is_volatile(&ofs->config))
> seq_puts(m, ",volatile");
> + else if (ofs->config.fsync_mode != ovl_fsync_mode_def())
> + seq_printf(m, ",fsync=%s",
> + ovl_fsync_mode(&ofs->config));
> if (ofs->config.userxattr)
> seq_puts(m, ",userxattr");
> if (ofs->config.verity_mode != ovl_verity_mode_def())
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 06a231970cb5..824cbcf40523 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -750,7 +750,7 @@ static int ovl_make_workdir(struct super_block *sb, struct ovl_fs *ofs,
> * For volatile mount, create a incompat/volatile/dirty file to keep
> * track of it.
> */
> - if (ofs->config.ovl_volatile) {
> + if (ovl_is_volatile(&ofs->config)) {
> err = ovl_create_volatile_dirty(ofs);
> if (err < 0) {
> pr_err("Failed to create volatile/dirty file.\n");
>
> base-commit: 0c3836482481200ead7b416ca80c68a29cfdaabd
> --
> 2.45.2
>

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Previous message: Artem Savkov: "[PATCH bpf-next v2] selftests/bpf: fix compilation failure when CONFIG_NET_FOU!=y"
In reply to: Fei Lv: "[PATCH V2] ovl: fsync after metadata copy-up via mount option "fsync=strict""
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