Message-ID: <6f4bcd7d79f688120d80e96e86d7c521854d8e84.camel@kernel.org>
Date: Wed, 21 Jun 2023 14:01:09 -0400
From: Jeff Layton <jlayton@...nel.org>
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Subject: Re: [PATCH 01/79] fs: add ctime accessors infrastructure

On Wed, 2023-06-21 at 13:29 -0400, Tom Talpey wrote:
> On 6/21/2023 10:45 AM, Jeff Layton wrote:
> > struct timespec64 has unused bits in the tv_nsec field that can be used
> > for other purposes. In future patches, we're going to change how the
> > inode->i_ctime is accessed in certain inodes in order to make use of
> > them. In order to do that safely though, we'll need to eradicate raw
> > accesses of the inode->i_ctime field from the kernel.
> > 
> > Add new accessor functions for the ctime that we can use to replace them.
> > 
> > Signed-off-by: Jeff Layton <jlayton@...nel.org>
> > ---
> >   fs/inode.c         | 16 ++++++++++++++
> >   include/linux/fs.h | 53 +++++++++++++++++++++++++++++++++++++++++++++-
> >   2 files changed, 68 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/inode.c b/fs/inode.c
> > index d37fad91c8da..c005e7328fbb 100644
> > --- a/fs/inode.c
> > +++ b/fs/inode.c
> > @@ -2499,6 +2499,22 @@ struct timespec64 current_time(struct inode *inode)
> >   }
> >   EXPORT_SYMBOL(current_time);
> >   
> > +/**
> > + * inode_ctime_set_current - set the ctime to current_time
> > + * @inode: inode
> > + *
> > + * Set the inode->i_ctime to the current value for the inode. Returns
> > + * the current value that was assigned to i_ctime.
> > + */
> > +struct timespec64 inode_ctime_set_current(struct inode *inode)
> > +{
> > +	struct timespec64 now = current_time(inode);
> > +
> > +	inode_set_ctime(inode, now);
> > +	return now;
> > +}
> > +EXPORT_SYMBOL(inode_ctime_set_current);
> > +
> >   /**
> >    * 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 6867512907d6..9afb30606373 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -1474,7 +1474,58 @@ static inline bool fsuidgid_has_mapping(struct super_block *sb,
> >   	       kgid_has_mapping(fs_userns, kgid);
> >   }
> >   
> > -extern struct timespec64 current_time(struct inode *inode);
> > +struct timespec64 current_time(struct inode *inode);
> > +struct timespec64 inode_ctime_set_current(struct inode *inode);
> > +
> > +/**
> > + * inode_ctime_peek - fetch the current ctime from the inode
> > + * @inode: inode from which to fetch ctime
> > + *
> > + * Grab the current ctime from the inode and return it.
> > + */
> > +static inline struct timespec64 inode_ctime_peek(const struct inode *inode)
> > +{
> > +	return inode->i_ctime;
> > +}
> > +
> > +/**
> > + * inode_ctime_set - set the ctime in the inode to the given value
> > + * @inode: inode in which to set the ctime
> > + * @ts: timespec value to set the ctime
> > + *
> > + * Set the ctime in @inode to @ts.
> > + */
> > +static inline struct timespec64 inode_ctime_set(struct inode *inode, struct timespec64 ts)
> > +{
> > +	inode->i_ctime = ts;
> > +	return ts;
> > +}
> > +
> > +/**
> > + * inode_ctime_set_sec - set only the tv_sec field in the inode ctime
> 
> I'm curious about why you choose to split the tv_sec and tv_nsec
> set_ functions. Do any callers not set them both? Wouldn't a
> single call enable a more atomic behavior someday?
> 
>    inode_ctime_set_sec_nsec(struct inode *, time64_t, time64_t)
> 
> (or simply initialize a timespec64 and use inode_ctime_spec() )
> 

Yes, quite a few places set the fields individually. For example, when
loading a value from disk that doesn't have sufficient granularity to
set the nsecs field to anything but 0.

Could I have done it by declaring a local timespec64 variable and just
use the inode_ctime_set function in these places? Absolutely.

That's a bit more difficult to handle with coccinelle though. If someone
wants to suggest a way to do that without having to change all of these
call sites manually, then I'm open to redoing the set.

That might be better left for a later cleanup though.

> > + * @inode: inode in which to set the ctime
> > + * @sec:  value to set the tv_sec field
> > + *
> > + * Set the sec field in the ctime. Returns @sec.
> > + */
> > +static inline time64_t inode_ctime_set_sec(struct inode *inode, time64_t sec)
> > +{
> > +	inode->i_ctime.tv_sec = sec;
> > +	return sec;
> > +}
> > +
> > +/**
> > + * inode_ctime_set_nsec - set only the tv_nsec field in the inode ctime
> > + * @inode: inode in which to set the ctime
> > + * @nsec:  value to set the tv_nsec field
> > + *
> > + * Set the nsec field in the ctime. Returns @nsec.
> > + */
> > +static inline long inode_ctime_set_nsec(struct inode *inode, long nsec)
> > +{
> > +	inode->i_ctime.tv_nsec = nsec;
> > +	return nsec;
> > +}
> >   
> >   /*
> >    * Snapshotting support.

-- 
Jeff Layton <jlayton@...nel.org>

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