Re: NFS file size anomaly?

[Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>]

On Wed 11-12-13 10:01:43, Dan Duval wrote:
> 
> ----- jack-AlSwsSmVLrQ@public.gmane.org wrote:
> 
> > On Tue 10-12-13 19:01:21, Trond Myklebust wrote:
> > > On Tue, 2013-12-10 at 15:36 +0100, Jan Kara wrote:
> > > > On Mon 09-12-13 19:56:16, Chuck Lever wrote:
> > > > > On Dec 9, 2013, at 4:05 PM, Chuck Lever <chuck.lever-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
> > wrote:
> > > > > > Copying Trond's new work address.
> > > > > > 
> > > > > > Begin forwarded message:
> > > > > > 
> > > > > >> From: Dan Duval <dan.duval-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
> > > > > >> Subject: NFS file size anomaly?
> > > > > >> Date: December 9, 2013 3:04:27 PM EST
> > > > > >> To: Trond Myklebust <Trond.Myklebust-HgOvQuBEEgTQT0dZR+AlfA@public.gmane.org>
> > > > > >> Cc: <linux-nfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
> > <linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
> > > > > >> 
> > > > > >> [NOTE: cross-posted to linux-nfs and linux-fsdevel]
> > > > > >> 
> > > > > >> I'm seeing some unexpected behavior with NFS and file sizes.
> > > > > >> 
> > > > > >> The test cases are from the LTP (Linux Test Project), tests
> > > > > >> ftest01, ftest05, and ftest07.  I'll concentrate on ftest01
> > > > > >> to explain what I've found.
> > > > > >> 
> > > > > >> ftest01 fires off 5 subprocesses, each of which opens an
> > empty
> > > > > >> file and does the following, repeatedly:
> > > > > >> 
> > > > > >>       . lseek to some point in the file
> > > > > >>       . read 2048 bytes
> > > > > >>       . lseek back to the same point
> > > > > >>       . write 2048 bytes
> > > > > >> 
> > > > > >> The "point in the file" is determined by a pseudo-random
> > > > > >> sequence.  All such points are on 2048-byte boundaries.
> > > > > >> 
> > > > > >> Occasionally, also driven pseudo-randomly, ftest01 will
> > throw
> > > > > >> in a call to ftruncate(), truncate(), sync(), or fstat().
> > > > > >> 
> > > > > >> With the fstat() calls, the returned .st_size is compared
> > > > > >> with the test's expected size for the file, and an error is
> > > > > >> declared if they don't match.
> > > > > >> 
> > > > > >> What's happening is that, some way into the test, this
> > fstat()
> > > > > >> check is failing.  Specifically, the .st_size reported by
> > > > > >> fstat() is greater than the computed size.
> > > > > >> 
> > > > > >> The sequence of operations leading up to this is:
> > > > > >> 
> > > > > >>       lseek 1034240 0
> > > > > >>       read 2048
> > > > > >>       lseek 0 1
> > > > > >>       write 2048
> > > > > >> 
> > > > > >>       lseek 638976 0
> > > > > >>       (read, lseek, write)
> > > > > >> 
> > > > > >>       lseek 708608 0
> > > > > >>       (read, lseek, write)
> > > > > >> 
> > > > > >>       lseek 708608 0
> > > > > >>       (read, lseek, write)
> > > > > >> 
> > > > > >>       lseek 679584 0
> > > > > >>       (read, lseek, write)
> > > > > >> 
> > > > > >>       truncate 266240
> > > > > >> 
> > > > > >>       lseek 960512 0
> > > > > >>       (read, lseek, write)
> > > > > >> 
> > > > > >>       (a bunch of lseek/read/lseek/write ops that do not
> > > > > >>        extend the file)
> > > > > >> 
> > > > > >>       fstat
> > > > > >> 
> > > > > >> So the expected size of the file is 960512 + 2048 == 960560.
> > > > > >> But the fstat reports a size of 1036288.
> > > > > >> 
> > > > > >> A look at what's happening on the wire, distilled from the
> > > > > >> output of tethereal, is instructive.
> > > > > >> 
> > > > > >>       READ Call 638976 4096 (byte offset and size to read)
> > > > > >>       READ Reply 4096 995382 (bytes read and current file
> > size)
> > > > > >> 
> > > > > >>       SETATTR Call 266240 (this corresponds to the truncate()
> > call)
> > > > > >> 
> > > > > >>       WRITE Call 638976 4096 (byte offset and size to write)
> > > > > >>       WRITE Call 708608 4096
> > > > > >>       WRITE Call 1032192 4096
> > > > > >> 
> > > > > >>       SETATTR Reply 266240 (current size of file)
> > > > > >> 
> > > > > >>       WRITE Reply 643072 (current size of file after write)
> > > > > >>       WRITE Reply 1036288
> > > > > >>       WRITE Reply 1036288
> > > > > >> 
> > > > > >>       GETATTR (initiated internally by NFS code?)
> > > > > >> 
> > > > > >>       READ Call 958464 4096 READ Reply 4096 1036288
> > > > > >> 
> > > > > >>       ... (a bunch of READ and WRITE ops that do not extend
> > the file)
> > > > > >> 
> > > > > >>       GETATTR Call (this corresponds to the fstat() call)
> > > > > >>       GETATTR Reply 1036288
> > > > > >> 
> > > > > >> So what appears to have happened here is that three of the
> > > > > >> WRITE operations that the program issued before the
> > truncate()
> > > > > >> call have "bled past" the SETATTR, extending the file
> > further
> > > > > >> than the SETATTR did.  Since none of the operations issued
> > > > > >> after SETATTR extends the file further, by the time we get
> > to
> > > > > >> the GETATTR, the file is larger than the test expects.
> > > > > >> 
> > > > > >> There are two strange things going on here.  The first,
> > > > > >> identified above, is that write()s that were initiated
> > before
> > > > > >> the truncate() call are being processed after the resulting
> > > > > >> SETATTR call.  The second is that WRITE operations are being
> > > > > >> initiated while the SETATTR is outstanding.
> > > > > >> 
> > > > > >> It seems to me that a size-changing SETATTR operation should
> > > > > >> act essentially as an I/O barrier. It should wait for all
> > outstanding
> > > > > >> read/write requests to complete, then issue the SETATTR,
> > > > > >> wait for the reply, and only then re-enable read/write
> > requests.
> > > > > >> 
> > > > > >> In other words, SETATTR should be atomic with respect to
> > other
> > > > > >> I/O operations.
> > > > > 
> > > > > nfs_setattr() relies on writeback_single_inode() [ via
> > nfs_setattr() ->
> > > > > nfs_wb_all() -> sync_inode() -> writeback_single_inode() ] to
> > serialize
> > > > > itself with other pending I/O operations.  The NFS client
> > already treats
> > > > > SETATTR as an I/O barrier. 
> > > > > 
> > > > > >> A git bisect indicates that this problem first appeared (or
> > > > > >> was first uncovered) with this commit:
> > > > > >> 
> > > > > >>   4f8ad65 writeback: Refactor writeback_single_inode()
> > > > > >> 
> > > > > >> It continues to the most recent mainline kernels.
> > > > > >> 
> > > > > >> NFS v3 vs. v4 doesn't seem to matter.
> > > > > >> 
> > > > > >> Has anyone else seen this?  Any pointers you can provide?
> > > > > 
> > > > > After commit 4f8ad65, writeback_single_inode() occasionally
> > returns
> > > > > immediately instead of waiting for writeback to finish.  The
> > behavior
> > > > > would be difficult or impossible to spot with a local
> > filesystem, but
> > > > > with NFS, a race is clearly exposed.
> > > >   That looks like a bug. But the only thing that's changed by
> > 4f8ad65 is
> > > > the additional test:
> > > > 	if (!(inode->i_state & I_DIRTY))
> > > > 		goto out;
> > > > So it seems NFS has inodes that have dirty data but don't have
> > (any of) inode
> > > > dirty bits set? But I've been staring at the code and don't see
> > any
> > > > problem. Any idea Chuck? Otherwise I guess I'll have to reproduce
> > this and
> > > > debug what's going on.
> > > 
> > > Hi Jan,
> > > 
> > > I'm guessing that the problem is the fact that you can clear the
> > > I_DIRTY_PAGES flag inside __writeback_single_inode() on a
> > non-waiting
> > > call (i.e. one which does not have WB_SYNC_ALL set).
> > > When that happens, the NFS client ends up with inode->i_state == 0
> > until
> > > one of the WRITEs returns with the "unstable" flag set (which
> > triggers a
> > > call to __mark_inode_dirty(inode, I_DIRTY_DATASYNC) so that we call
> > > write_inode()).
> > > 
> > > Basically, if you have WB_SYNC_ALL set, I think that you cannot
> > apply
> > > the test for (inode->i_state & I_DIRTY) until you have called 
> > > filemap_fdatawait().
> >   Ah, thanks for explanation. I should have spotted this myself. So
> > something like the attached patch should fix the issue. Dan, can you
> > try
> > whether the attached patch fixes your problem?
> > 
> > 								Honza
> > -- 
> > Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>
> > SUSE Labs, CR
> 
> Jan,
> 
> Confirmed - this patch eliminates the errors I was seeing.  The LTP tests
> now pass reliably.
  Thanks for confirmation. I've submitted the patch for inclusion.

								Honza

-- 
Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>
SUSE Labs, CR
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