Re: [PATCH RFC v3] vfs: make fstatat retry once on ESTALE errors from getattr call

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On Wed, 18 Apr 2012 07:52:07 -0400
Jeff Layton <jlayton@xxxxxxxxxx> wrote:

> ESTALE errors are a source of pain for many users of NFS. Usually they
> occur when a file is removed from the server after a successful lookup
> against it.
> 
> Luckily, the remedy in these cases is usually simple. We should just
> redo the lookup, forcing revalidations all the way in and then retry the
> call. We of course cannot do this for syscalls that do not involve a
> path, but for path-based syscalls we can and should attempt to recover
> from an ESTALE.
> 
> This patch implements this by having the VFS reattempt the lookup (with
> LOOKUP_REVAL set) and call exactly once when it would ordinarily return
> ESTALE. This should catch the bulk of these cases under normal usage,
> without unduly inconveniencing other filesystems that return ESTALE on
> path-based syscalls.
> 
> Note that it's possible to hit this race more than once, but a single
> retry should catch the bulk of these cases under normal circumstances.
> 
> This patch is just an example. We'll alter most path-based syscalls in a
> similar fashion to fix this correctly. At this point, I'm just trying to
> ensure that the retry semantics are acceptable before I being that work.
> 
> Does anyone have strong objections to this patch? I'm aware that the
> retry mechanism is not as robust as many (e.g. Peter) would like, but it
> should at least improve the current situation.
> 
> If no one has a strong objection, then I'll start going through and
> adding similar code to the other syscalls. And we can hopefully we can
> get at least some of them in for 3.5.
> 
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
>  fs/stat.c |    9 ++++++++-
>  1 files changed, 8 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/stat.c b/fs/stat.c
> index c733dc5..0ee9cb4 100644
> --- a/fs/stat.c
> +++ b/fs/stat.c
> @@ -73,7 +73,8 @@ int vfs_fstatat(int dfd, const char __user *filename, struct kstat *stat,
>  {
>  	struct path path;
>  	int error = -EINVAL;
> -	int lookup_flags = 0;
> +	bool retried = false;
> +	unsigned int lookup_flags = 0;
>  
>  	if ((flag & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT |
>  		      AT_EMPTY_PATH)) != 0)
> @@ -84,12 +85,18 @@ int vfs_fstatat(int dfd, const char __user *filename, struct kstat *stat,
>  	if (flag & AT_EMPTY_PATH)
>  		lookup_flags |= LOOKUP_EMPTY;
>  
> +retry:
>  	error = user_path_at(dfd, filename, lookup_flags, &path);
>  	if (error)
>  		goto out;
>  
>  	error = vfs_getattr(path.mnt, path.dentry, stat);
>  	path_put(&path);
> +	if (error == -ESTALE && !retried) {
> +		retried = true;
> +		lookup_flags |= LOOKUP_REVAL;
> +		goto retry;
> +	}
>  out:
>  	return error;
>  }

Apologies for replying to myself here. Just to beat on the deceased
equine a little longer, I should note that the above approach does
*not* fix Peter's reproducer in his original email. It fails rather
quickly when run simultaneously on the client and server.

At least one of the tests in it creates and removes a hierarchy of
directories in a loop. During that, the lookup from the client can
easily fail more than once with ESTALE. Since we give up after a single
retry, that causes the call to return ESTALE.

While testing this approach with mkdir and fstatat, I modified the
patch to retry multiple times, also retry when the lookup thows ESTALE
and to throw a printk when the number of retries was > 1 with the
number of retries that the call did and the eventual error code.

With Peter's (admittedly synthetic) test, we can get an answer of sorts
to Trond's question from earlier in the thread as to how many retries
is "enough":

[   45.023665] sys_mkdirat: retries=33 error=-2
[   47.889300] sys_mkdirat: retries=51 error=-2
[   49.172746] sys_mkdirat: retries=27 error=-2
[   52.325723] sys_mkdirat: retries=10 error=-2
[   58.082576] sys_mkdirat: retries=33 error=-2
[   59.810461] sys_mkdirat: retries=5 error=-2
[   63.387914] sys_mkdirat: retries=14 error=-2
[   63.630785] sys_mkdirat: retries=4 error=-2
[   68.268903] sys_mkdirat: retries=6 error=-2
[   71.124173] sys_mkdirat: retries=99 error=-2
[   75.657649] sys_mkdirat: retries=123 error=-2
[   76.903814] sys_mkdirat: retries=26 error=-2
[   82.009463] sys_mkdirat: retries=30 error=-2
[   84.807731] sys_mkdirat: retries=67 error=-2
[   89.825529] sys_mkdirat: retries=166 error=-2
[   91.599104] sys_mkdirat: retries=8 error=-2
[   95.621855] sys_mkdirat: retries=44 error=-2
[   98.164588] sys_mkdirat: retries=61 error=-2
[   99.783347] sys_mkdirat: retries=11 error=-2
[  105.593980] sys_mkdirat: retries=104 error=-2
[  110.348861] sys_mkdirat: retries=8 error=-2
[  112.087966] sys_mkdirat: retries=46 error=-2
[  117.613316] sys_mkdirat: retries=90 error=-2
[  120.117550] sys_mkdirat: retries=2 error=-2
[  122.624330] sys_mkdirat: retries=15 error=-2

So, now I'm having buyers remorse of sorts about proposing to just
retry once as that may not be strong enough to fix some of the cases
we're interested in.

I guess the questions at this point is:

1) How representative is Peter's mkdir_test() of a real-world workload?

2) if we assume that it is fairly representative of one, how can we
achieve retrying indefinitely with NFS, or at least some large finite
amount?

I have my doubts as to whether it would really be as big a problem for
other filesystems as Miklos and others have asserted, but I'll take
their word for it at the moment. What's the best way to contain this
behavior to just those filesystems that want to retry indefinitely when
they get an ESTALE? Would we need to go with an entirely new
ESTALERETRY after all?

-- 
Jeff Layton <jlayton@xxxxxxxxxx>
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