Re: [PATCH] tmpfs: add support for an i_version counter

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2022-09-09 at 14:03 -0700, Andrew Morton wrote:
> On Fri,  9 Sep 2022 09:00:31 -0400 Jeff Layton <jlayton@kernel.org> wrote:
> 
> > NFSv4 mandates a change attribute to avoid problems with timestamp
> > granularity, which Linux implements using the i_version counter. This is
> > particularly important when the underlying filesystem is fast.
> > 
> > Give tmpfs an i_version counter. Since it doesn't have to be persistent,
> > we can just turn on SB_I_VERSION and sprinkle some inode_inc_iversion
> > calls in the right places.
> > 
> > Also, while there is no formal spec for xattrs, most implementations
> > update the ctime on setxattr. Fix shmem_xattr_handler_set to update the
> > ctime and bump the i_version appropriately.
> > 
> > ...
> > 
> > --- a/fs/posix_acl.c
> > +++ b/fs/posix_acl.c
> > @@ -24,6 +24,7 @@
> >  #include <linux/user_namespace.h>
> >  #include <linux/namei.h>
> >  #include <linux/mnt_idmapping.h>
> > +#include <linux/iversion.h>
> >  
> >  static struct posix_acl **acl_by_type(struct inode *inode, int type)
> >  {
> > @@ -1073,6 +1074,8 @@ int simple_set_acl(struct user_namespace *mnt_userns, struct inode *inode,
> >  	}
> >  
> >  	inode->i_ctime = current_time(inode);
> > +	if (IS_I_VERSION(inode))
> > +		inode_inc_iversion(inode);
> >  	set_cached_acl(inode, type, acl);
> >  	return 0;
> >  }
> 
> adds a kilobyte of text to shmem.o because the quite large
> inode_maybe_inc_iversion() get inlined all over the place.  Why oh why.
> 
> Is there any reason not to do the obvious?
> 

No reason at all:

Acked-by: Jeff Layton <jlayton@kernel.org>

> --- a/include/linux/iversion.h~a
> +++ a/include/linux/iversion.h
> @@ -177,56 +177,7 @@ inode_set_iversion_queried(struct inode
>  				I_VERSION_QUERIED);
>  }
>  
> -/**
> - * inode_maybe_inc_iversion - increments i_version
> - * @inode: inode with the i_version that should be updated
> - * @force: increment the counter even if it's not necessary?
> - *
> - * Every time the inode is modified, the i_version field must be seen to have
> - * changed by any observer.
> - *
> - * If "force" is set or the QUERIED flag is set, then ensure that we increment
> - * the value, and clear the queried flag.
> - *
> - * In the common case where neither is set, then we can return "false" without
> - * updating i_version.
> - *
> - * If this function returns false, and no other metadata has changed, then we
> - * can avoid logging the metadata.
> - */
> -static inline bool
> -inode_maybe_inc_iversion(struct inode *inode, bool force)
> -{
> -	u64 cur, old, new;
> -
> -	/*
> -	 * The i_version field is not strictly ordered with any other inode
> -	 * information, but the legacy inode_inc_iversion code used a spinlock
> -	 * to serialize increments.
> -	 *
> -	 * Here, we add full memory barriers to ensure that any de-facto
> -	 * ordering with other info is preserved.
> -	 *
> -	 * This barrier pairs with the barrier in inode_query_iversion()
> -	 */
> -	smp_mb();
> -	cur = inode_peek_iversion_raw(inode);
> -	for (;;) {
> -		/* If flag is clear then we needn't do anything */
> -		if (!force && !(cur & I_VERSION_QUERIED))
> -			return false;
> -
> -		/* Since lowest bit is flag, add 2 to avoid it */
> -		new = (cur & ~I_VERSION_QUERIED) + I_VERSION_INCREMENT;
> -
> -		old = atomic64_cmpxchg(&inode->i_version, cur, new);
> -		if (likely(old == cur))
> -			break;
> -		cur = old;
> -	}
> -	return true;
> -}
> -
> +bool inode_maybe_inc_iversion(struct inode *inode, bool force);
>  
>  /**
>   * inode_inc_iversion - forcibly increment i_version
> --- a/fs/libfs.c~a
> +++ a/fs/libfs.c
> @@ -15,6 +15,7 @@
>  #include <linux/mutex.h>
>  #include <linux/namei.h>
>  #include <linux/exportfs.h>
> +#include <linux/iversion.h>
>  #include <linux/writeback.h>
>  #include <linux/buffer_head.h> /* sync_mapping_buffers */
>  #include <linux/fs_context.h>
> @@ -1529,3 +1530,53 @@ void generic_set_encrypted_ci_d_ops(stru
>  #endif
>  }
>  EXPORT_SYMBOL(generic_set_encrypted_ci_d_ops);
> +
> +/**
> + * inode_maybe_inc_iversion - increments i_version
> + * @inode: inode with the i_version that should be updated
> + * @force: increment the counter even if it's not necessary?
> + *
> + * Every time the inode is modified, the i_version field must be seen to have
> + * changed by any observer.
> + *
> + * If "force" is set or the QUERIED flag is set, then ensure that we increment
> + * the value, and clear the queried flag.
> + *
> + * In the common case where neither is set, then we can return "false" without
> + * updating i_version.
> + *
> + * If this function returns false, and no other metadata has changed, then we
> + * can avoid logging the metadata.
> + */
> +bool inode_maybe_inc_iversion(struct inode *inode, bool force)
> +{
> +	u64 cur, old, new;
> +
> +	/*
> +	 * The i_version field is not strictly ordered with any other inode
> +	 * information, but the legacy inode_inc_iversion code used a spinlock
> +	 * to serialize increments.
> +	 *
> +	 * Here, we add full memory barriers to ensure that any de-facto
> +	 * ordering with other info is preserved.
> +	 *
> +	 * This barrier pairs with the barrier in inode_query_iversion()
> +	 */
> +	smp_mb();
> +	cur = inode_peek_iversion_raw(inode);
> +	for (;;) {
> +		/* If flag is clear then we needn't do anything */
> +		if (!force && !(cur & I_VERSION_QUERIED))
> +			return false;
> +
> +		/* Since lowest bit is flag, add 2 to avoid it */
> +		new = (cur & ~I_VERSION_QUERIED) + I_VERSION_INCREMENT;
> +
> +		old = atomic64_cmpxchg(&inode->i_version, cur, new);
> +		if (likely(old == cur))
> +			break;
> +		cur = old;
> +	}
> +	return true;
> +}
> +EXPORT_SYMBOL(inode_maybe_inc_iversion);
> _
> 

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