review: don't block non-blocking writes when frozen

review: don't block non-blocking writes when frozen

[David Chinner]

Prevent nfsds for blocking trying to write to a frozen filesystem
or a filesystem in the process of freezing.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group

---
 fs/xfs/linux-2.6/xfs_lrw.c |    4 ++++
 1 file changed, 4 insertions(+)

Index: 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_lrw.c
===================================================================
--- 2.6.x-xfs-new.orig/fs/xfs/linux-2.6/xfs_lrw.c	2007-03-29 19:03:30.000000000 +1000
+++ 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_lrw.c	2007-03-29 19:08:06.262169809 +1000
@@ -684,6 +684,10 @@ xfs_write(
 	io = &xip->i_iocore;
 	mp = io->io_mount;
 
+	if (FILP_DELAY_FLAG(file) && vfs_test_for_freeze(vp->v_vfsp)) {
+		/* so nfsd can return EJUKEBOX to clients during a freeze */
+		return -EAGAIN;
+	}
 	vfs_wait_for_freeze(vp->v_vfsp, SB_FREEZE_WRITE);
 
 	if (XFS_FORCED_SHUTDOWN(mp))

Re: review: don't block non-blocking writes when frozen

[Christoph Hellwig]

On Mon, Apr 23, 2007 at 10:26:16AM +1000, David Chinner wrote:
> 
> Prevent nfsds for blocking trying to write to a frozen filesystem
> or a filesystem in the process of freezing.

Looks good for trees actually having support for non-blocking file
I/O, which doesn't include mainline.  (So please don't send this
upstead)

Re: review: don't block non-blocking writes when frozen

[David Chinner]

On Mon, Apr 23, 2007 at 10:27:15PM +0100, Christoph Hellwig wrote:
> On Mon, Apr 23, 2007 at 10:26:16AM +1000, David Chinner wrote:
> > 
> > Prevent nfsds for blocking trying to write to a frozen filesystem
> > or a filesystem in the process of freezing.
> 
> Looks good for trees actually having support for non-blocking file
> I/O, which doesn't include mainline.  (So please don't send this
> upstead)

Yeah - you NACKed that a year ago based on the fact it would never
get used by mainline code:

http://marc.info/?l=linux-nfs&m=114683005119982&w=2

So, given the catch-22 you've just presented us can we revisit the
nfsd non-blocking I/O issue again?  This affects anyone using DM
snapshots on their NFS servers and has nothing to do with HSMs
or DMAPI...

FWIW, you can still do non-blocking userspace I/O to a file, so this
XFS patch is still valid for mainline (that's how I tested it).

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group

Re: review: don't block non-blocking writes when frozen

[Christoph Hellwig]

On Tue, Apr 24, 2007 at 09:13:37AM +1000, David Chinner wrote:
> So, given the catch-22 you've just presented us can we revisit the
> nfsd non-blocking I/O issue again?  This affects anyone using DM
> snapshots on their NFS servers and has nothing to do with HSMs
> or DMAPI...
> 
> FWIW, you can still do non-blocking userspace I/O to a file, so this
> XFS patch is still valid for mainline (that's how I tested it).

I've been investigating the situation a little bit more, and here's
what's going on:

 - SuS allows for O_NONBLOCK on regular files as per
   http://www.opengroup.org/onlinepubs/007908799/xsh/write.html
 - actually implementing O_NONBLOCK semantics for regular fixes
   breaks userspace when poll/select claims files are ready to
   read/write but they aren't, see http://lkml.org/lkml/2004/10/17/17

So we can't really expose O_NONBLOCK on regular files to userspace,
and we need to make sure in common code this does not happen.

EJUKEBOX on snaphots does make sense, though.  Can you please send
a full patchseries for nfsd, the common code and the xfs writepath
so that this actually gets used and behaviour is consistant for all
(or at least most) filesystems?

Also now that the patch goes to mainline please kill ugly FILP_DELAY_FLAG
and just check the flags directly.  And it should probably only check
O_NONBLOCK.  The only architecture having O_NDELAY different from
O_NONBLOCK is sparc, and it already translates the value for us.