Re: recent intermittent fsx-related failures

[Chuck Lever III <chuck.lever@oracle.com>]

> On Sep 21, 2021, at 3:00 PM, Trond Myklebust <trondmy@hammerspace.com> wrote:
> 
> On Sun, 2021-09-19 at 23:03 +0000, Chuck Lever III wrote:
>> 
>> 
>>> On Jul 23, 2021, at 4:24 PM, Trond Myklebust
>>> <trondmy@hammerspace.com> wrote:
>>> 
>>> On Fri, 2021-07-23 at 20:12 +0000, Chuck Lever III wrote:
>>>> Hi-
>>>> 
>>>> I noticed recently that generic/075, generic/112, and generic/127
>>>> were
>>>> failing intermittently on NFSv3 mounts. All three of these tests
>>>> are
>>>> based on fsx.
>>>> 
>>>> "git bisect" landed on this commit:
>>>> 
>>>> 7b24dacf0840 ("NFS: Another inode revalidation improvement")
>>>> 
>>>> After reverting 7b24dacf0840 on v5.14-rc1, I can no longer
>>>> reproduce
>>>> the test failures.
>>>> 
>>>> 
>>> 
>>> So you are seeing file metadata updates that end up not changing
>>> the
>>> ctime?
>> 
>> As far as I can tell, a WRITE and two SETATTRs are happening in
>> sequence to the same file during the same jiffy. The WRITE does
>> not report pre/post attributes, but the SETATTRs do. The reported
>> pre- and post- mtime and ctime are all the same value for both
>> SETATTRs, I believe due to timestamp_truncate().
>> 
>> My theory is that persistent-storage-backed filesystems seem to
>> go slow enough that it doesn't become a significant problem. But
>> with tmpfs, this can happen often enough that the client gets
>> confused. And I can make the problem unreproducable if I enable
>> enough debugging paraphernalia on the server to slow it down.
>> 
>> I'm not exactly sure how the client becomes confused by this
>> behavior, but fsx reports a stale size value, or it can hit a
>> bus error. I'm seeing at least four of the fsx-based xfs tests
>> fail intermittently.
>> 
> 
> The client no longer relies on post-op attributes in order to update
> the metadata after a successful SETATTR. If you look at
> nfs_setattr_update_inode() you'll see that it picks the values that
> were set directly from the iattr argument.
> 
> The post-op attributes are only used to determine the implicit
> timestamp updates, and to detect any other updates that may have
> happened.

I've been able to directly and repeatedly observe the size attribute
reverting to a previous value.

The issue stems from the MM driving a background readahead operation
at the same time the application truncates or extends the file. The
READ starts before the size-mutating operation and completes after
it.

If the server happens to have done the READ before the size-mutating
operation, the READ result contains the previous size value. When
the READ completes, the client overwrites the more recent size
value with the stale one.

I'm not yet sure how this relates to

7b24dacf0840 ("NFS: Another inode revalidation improvement")

and maybe it doesn't. "git bisect" with an unreliable reproducer
generates notoriously noisy data. 


--
Chuck Lever