Message-ID: <20241001133219.aenkpa5ydeefqqyd@quack3>
Date: Tue, 1 Oct 2024 15:32:19 +0200
From: Jan Kara <jack@...e.cz>
To: Jeff Layton <jlayton@...nel.org>
Cc: John Stultz <jstultz@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
	Stephen Boyd <sboyd@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Jonathan Corbet <corbet@....net>,
	Randy Dunlap <rdunlap@...radead.org>,
	Chandan Babu R <chandan.babu@...cle.com>,
	"Darrick J. Wong" <djwong@...nel.org>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>, Chris Mason <clm@...com>,
	Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chuck Lever <chuck.lever@...cle.com>,
	Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-btrfs@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH v8 04/12] fs: handle delegated timestamps in
 setattr_copy_mgtime

On Tue 01-10-24 06:58:58, Jeff Layton wrote:
> When updating the ctime on an inode for a SETATTR with a multigrain
> filesystem, we usually want to take the latest time we can get for the
> ctime. The exception to this rule is when there is a nfsd write
> delegation and the server is proxying timestamps from the client.
> 
> When nfsd gets a CB_GETATTR response, we want to update the timestamp
> value in the inode to the values that the client is tracking. The client
> doesn't send a ctime value (since that's always determined by the
> exported filesystem), but it can send a mtime value. In the case where
> it does, then we may need to update the ctime to a value commensurate
> with that instead of the current time.
> 
> If ATTR_DELEG is set, then use ia_ctime value instead of setting the
> timestamp to the current time.
> 
> With the addition of delegated timestamps we can also receive a request
> to update only the atime, but we may not need to set the ctime. Trust
> the ATTR_CTIME flag in the update and only update the ctime when it's
> set.
> 
> Tested-by: Randy Dunlap <rdunlap@...radead.org> # documentation bits
> Signed-off-by: Jeff Layton <jlayton@...nel.org>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/attr.c          | 28 +++++++++++++--------
>  fs/inode.c         | 72 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/fs.h |  2 ++
>  3 files changed, 92 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/attr.c b/fs/attr.c
> index 3bcbc45708a3..392eb62aa609 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -286,16 +286,20 @@ static void setattr_copy_mgtime(struct inode *inode, const struct iattr *attr)
>  	unsigned int ia_valid = attr->ia_valid;
>  	struct timespec64 now;
>  
> -	/*
> -	 * If the ctime isn't being updated then nothing else should be
> -	 * either.
> -	 */
> -	if (!(ia_valid & ATTR_CTIME)) {
> -		WARN_ON_ONCE(ia_valid & (ATTR_ATIME|ATTR_MTIME));
> -		return;
> +	if (ia_valid & ATTR_CTIME) {
> +		/*
> +		 * In the case of an update for a write delegation, we must respect
> +		 * the value in ia_ctime and not use the current time.
> +		 */
> +		if (ia_valid & ATTR_DELEG)
> +			now = inode_set_ctime_deleg(inode, attr->ia_ctime);
> +		else
> +			now = inode_set_ctime_current(inode);
> +	} else {
> +		/* If ATTR_CTIME isn't set, then ATTR_MTIME shouldn't be either. */
> +		WARN_ON_ONCE(ia_valid & ATTR_MTIME);
>  	}
>  
> -	now = inode_set_ctime_current(inode);
>  	if (ia_valid & ATTR_ATIME_SET)
>  		inode_set_atime_to_ts(inode, attr->ia_atime);
>  	else if (ia_valid & ATTR_ATIME)
> @@ -354,8 +358,12 @@ void setattr_copy(struct mnt_idmap *idmap, struct inode *inode,
>  		inode_set_atime_to_ts(inode, attr->ia_atime);
>  	if (ia_valid & ATTR_MTIME)
>  		inode_set_mtime_to_ts(inode, attr->ia_mtime);
> -	if (ia_valid & ATTR_CTIME)
> -		inode_set_ctime_to_ts(inode, attr->ia_ctime);
> +	if (ia_valid & ATTR_CTIME) {
> +		if (ia_valid & ATTR_DELEG)
> +			inode_set_ctime_deleg(inode, attr->ia_ctime);
> +		else
> +			inode_set_ctime_to_ts(inode, attr->ia_ctime);
> +	}
>  }
>  EXPORT_SYMBOL(setattr_copy);
>  
> diff --git a/fs/inode.c b/fs/inode.c
> index 4ec1e71e9a9d..7a324d999816 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2751,6 +2751,78 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
>  }
>  EXPORT_SYMBOL(inode_set_ctime_current);
>  
> +/**
> + * inode_set_ctime_deleg - try to update the ctime on a delegated inode
> + * @inode: inode to update
> + * @update: timespec64 to set the ctime
> + *
> + * Attempt to atomically update the ctime on behalf of a delegation holder.
> + *
> + * The nfs server can call back the holder of a delegation to get updated
> + * inode attributes, including the mtime. When updating the mtime we may
> + * need to update the ctime to a value at least equal to that.
> + *
> + * This can race with concurrent updates to the inode, in which
> + * case we just don't do the update.
> + *
> + * Note that this works even when multigrain timestamps are not enabled,
> + * so use it in either case.
> + */
> +struct timespec64 inode_set_ctime_deleg(struct inode *inode, struct timespec64 update)
> +{
> +	struct timespec64 now, cur_ts;
> +	u32 cur, old;
> +
> +	/* pairs with try_cmpxchg below */
> +	cur = smp_load_acquire(&inode->i_ctime_nsec);
> +	cur_ts.tv_nsec = cur & ~I_CTIME_QUERIED;
> +	cur_ts.tv_sec = inode->i_ctime_sec;
> +
> +	/* If the update is older than the existing value, skip it. */
> +	if (timespec64_compare(&update, &cur_ts) <= 0)
> +		return cur_ts;
> +
> +	ktime_get_coarse_real_ts64_mg(&now);
> +
> +	/* Clamp the update to "now" if it's in the future */
> +	if (timespec64_compare(&update, &now) > 0)
> +		update = now;
> +
> +	update = timestamp_truncate(update, inode);
> +
> +	/* No need to update if the values are already the same */
> +	if (timespec64_equal(&update, &cur_ts))
> +		return cur_ts;
> +
> +	/*
> +	 * Try to swap the nsec value into place. If it fails, that means
> +	 * we raced with an update due to a write or similar activity. That
> +	 * stamp takes precedence, so just skip the update.
> +	 */
> +retry:
> +	old = cur;
> +	if (try_cmpxchg(&inode->i_ctime_nsec, &cur, update.tv_nsec)) {
> +		inode->i_ctime_sec = update.tv_sec;
> +		mgtime_counter_inc(mg_ctime_swaps);
> +		return update;
> +	}
> +
> +	/*
> +	 * Was the change due to someone marking the old ctime QUERIED?
> +	 * If so then retry the swap. This can only happen once since
> +	 * the only way to clear I_CTIME_QUERIED is to stamp the inode
> +	 * with a new ctime.
> +	 */
> +	if (!(old & I_CTIME_QUERIED) && (cur == (old | I_CTIME_QUERIED)))
> +		goto retry;
> +
> +	/* Otherwise, it was a new timestamp. */
> +	cur_ts.tv_sec = inode->i_ctime_sec;
> +	cur_ts.tv_nsec = cur & ~I_CTIME_QUERIED;
> +	return cur_ts;
> +}
> +EXPORT_SYMBOL(inode_set_ctime_deleg);
> +
>  /**
>   * in_group_or_capable - check whether caller is CAP_FSETID privileged
>   * @idmap:	idmap of the mount @inode was found from
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 23908bad166c..b1a3bd07711b 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1584,6 +1584,8 @@ static inline bool fsuidgid_has_mapping(struct super_block *sb,
>  
>  struct timespec64 current_time(struct inode *inode);
>  struct timespec64 inode_set_ctime_current(struct inode *inode);
> +struct timespec64 inode_set_ctime_deleg(struct inode *inode,
> +					struct timespec64 update);
>  
>  static inline time64_t inode_get_atime_sec(const struct inode *inode)
>  {
> 
> -- 
> 2.46.2
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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