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

From: Steve Dickson
Date: Mon Apr 23 2012 - 10:56:19 EST

On 04/20/2012 05:13 PM, Jeff Layton wrote:
> On Fri, 20 Apr 2012 16:18:37 -0400
> Steve Dickson <SteveD@xxxxxxxxxx> wrote:
>> On 04/20/2012 10:40 AM, Jeff Layton wrote:
>>> I guess the questions at this point is:
>>> 1) How representative is Peter's mkdir_test() of a real-world workload?
>> Reading your email I had to wonder the same thing... What application
>> removes hierarchy of directories in a loop from two different clients?
>> I would suspect not many, if any... esp over NFS...
> Peter's test just happens to demonstrate the problem well, but one
> could envision someone removing a heirarchy of directories on the
> server while we're trying to do other operations in it. At that point,
> we can easily end up hitting an ESTALE twice while doing the lookup and
> returning ESTALE back to userspace.
Just curious, what happens when you run Peter's mkdir_test() on a
local file system? Any errors returned?

I would think removing hierarchy of directories while they are being
accessed has to even cause local fs some type of havoc

>>> 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?
>> The amount of looping would be peer speculation. If the problem can
>> not be handled by one simple retry I would say we simply pass the
>> error up to the app... Its an application issue...
> It's not an application issue. The application just asked the kernel
> to do an operation on a pathname. The only reason you're getting an
> ESTALE back in this situation is a shortcoming of the implementation.
> We passed it a pathname after all, not a filehandle. ESTALE really has
> no place as a return code in that situation...
We'll have to agree to disagree... I think any application that is
removing hierarchies of file and directory w/out taking any
precautionary locking is a shortcoming of the application

>>> 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?
>> Introducing a new errno to handle this problem would be overkill IMHO...
>> If we have to go to the looping approach, I would strong suggest we
>> make the file systems register for this type of behavior...
> Returning ESTALERETRY would be registering for it in a way and it is
> somewhat cleaner than having to go all the way back up to the fstype to
> figure out whether you want to retry it or not.
How would legacy apps handle this new errno, esp if they have logic
to take care of ESTALE errors?

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