Re: [PATCH RFC] vfs: make fstatat retry on ESTALE errors fromgetattr call

[Jeff Layton]

On Tue, 17 Apr 2012 17:50:06 +0200
Miklos Szeredi <miklos@xxxxxxxxxx> wrote:

> Jeff Layton <jlayton@xxxxxxxxxx> writes:
> 
> > On Tue, 17 Apr 2012 16:27:16 +0200
> > Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> >
> >> Steve Dickson <SteveD@xxxxxxxxxx> writes:
> >> 
> >> > True, but even so... Giving file systems an opt-out option with the 
> >> > default being out, maybe still have some merit... Making file systems
> >> > enable this new type of functionality would cut down on any of the
> >> > "surprise" that might occur with this redo ;-) 
> >> 
> >> I've been arguing for something slightly different for quite some time:
> >> I never liked errno values which have side effects in the kernel yet
> >> might be visible to userspace.
> >> 
> >> So why not introduce ERETRYSTALE, a *kernel internal* errno value that
> >> userspace will never see and filesystems never accidentally set.  The
> >> VFS can turn this into ESTALE if it doesn't retry for some reason
> >> (e.g. already retried).
> >> 
> >
> > That's possible but it's certainly a lot more invasive. It's also far
> > more difficult for filesystems to opt-in to this sort of behavior.
> 
> As a first step all that is needed to opt in is to convert -ESTALE to
> -ESTALERETRY on those operations that allow retrying.  Which is lookup,
> at the moment.
> 
> If we decide to make more ops retryable (I'm not convinced that's the
> best approach, but it might be) then those can be converted without too
> much pain.
> 

We will need to make more operations retryable. The main problem is
that inodes can go stale between the lookup and the actual operation
that we want the syscall to do. Mostly, that happens when a lookup
is satisfied out of the dcache, but it can happen even after an
on-the-wire lookup too.

Just having an opt-in to to retry when the lookup fails won't
materially help that situation.

The more I look at it, the less fond I am of this new error code. The
patch below just increases the complexity of the lookup codepath by
introducing ERETRYLOOKUP, which basically means the same thing as
ESTALE in most situations. It doesn't really add much benefit that I
can see.

Again, what's the problem with just basing this decision on the existing
ESTALE return? Very few filesystems return ESTALE at all, and most of them
just do it in their export-ops codepaths which won't be involved here.
Do we really need to jump through these sorts of hoops in order to
avoid a single retry in an ESTALE codepath?

> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 0062dd1..66b7d06 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1776,9 +1776,14 @@ static int do_path_lookup(int dfd, const char *name,
>  	int retval = path_lookupat(dfd, name, flags | LOOKUP_RCU, nd);
>  	if (unlikely(retval == -ECHILD))
>  		retval = path_lookupat(dfd, name, flags, nd);
> -	if (unlikely(retval == -ESTALE))
> +	if (unlikely(retval == -ERETRYLOOKUP)) {
>  		retval = path_lookupat(dfd, name, flags | LOOKUP_REVAL, nd);
>  
> +		/* retry only once */
> +		if (retval == -ERETRYLOOKUP)
> +			retval = -ESTALE;
> +	}
> +
>  	if (likely(!retval)) {
>  		if (unlikely(!audit_dummy_context())) {
>  			if (nd->path.dentry && nd->inode)
> @@ -2433,8 +2438,12 @@ struct file *do_filp_open(int dfd, const char *pathname,
>  	filp = path_openat(dfd, pathname, &nd, op, flags | LOOKUP_RCU);
>  	if (unlikely(filp == ERR_PTR(-ECHILD)))
>  		filp = path_openat(dfd, pathname, &nd, op, flags);
> -	if (unlikely(filp == ERR_PTR(-ESTALE)))
> +	if (unlikely(filp == ERR_PTR(-ERETRYLOOKUP))) {
>  		filp = path_openat(dfd, pathname, &nd, op, flags | LOOKUP_REVAL);
> +		if (filp == ERR_PTR(-ERETRYLOOKUP))
> +			filp = ERR_PTR(-ESTALE);
> +	}
> +
>  	return filp;
>  }
>  
> @@ -2455,8 +2464,11 @@ struct file *do_file_open_root(struct dentry *dentry, struct vfsmount *mnt,
>  	file = path_openat(-1, name, &nd, op, flags | LOOKUP_RCU);
>  	if (unlikely(file == ERR_PTR(-ECHILD)))
>  		file = path_openat(-1, name, &nd, op, flags);
> -	if (unlikely(file == ERR_PTR(-ESTALE)))
> +	if (unlikely(file == ERR_PTR(-ESTALE))) {
>  		file = path_openat(-1, name, &nd, op, flags | LOOKUP_REVAL);
> +		if (file == ERR_PTR(-ERETRYLOOKUP))
> +			file = ERR_PTR(-ESTALE);
> +	}
>  	return file;
>  }
>  
> diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> index 4aaf031..cd35da3 100644
> --- a/fs/nfs/dir.c
> +++ b/fs/nfs/dir.c
> @@ -1303,6 +1303,8 @@ static struct dentry *nfs_lookup(struct inode *dir, struct dentry * dentry, stru
>  	if (error == -ENOENT)
>  		goto no_entry;
>  	if (error < 0) {
> +		if (error == -ESTALE)
> +			error = -ERETRYLOOKUP;
>  		res = ERR_PTR(error);
>  		goto out_unblock_sillyrename;
>  	}
> diff --git a/include/linux/errno.h b/include/linux/errno.h
> index 2d09bfa..cfaedb8 100644
> --- a/include/linux/errno.h
> +++ b/include/linux/errno.h
> @@ -17,6 +17,7 @@
>  #define ENOIOCTLCMD	515	/* No ioctl command */
>  #define ERESTART_RESTARTBLOCK 516 /* restart by calling sys_restart_syscall */
>  #define EPROBE_DEFER	517	/* Driver requests probe retry */
> +#define ERETRYLOOKUP	519	/* Retry lookup */
>  
>  /* Defined for the NFSv3 protocol */
>  #define EBADHANDLE	521	/* Illegal NFS file handle */
> 
> 
> 


-- 
Jeff Layton <jlayton@xxxxxxxxxx>
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