Date: Tue, 21 Sep 1999 05:26:16 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: "Justin T. Gibbs" <gibbs@caspian.plutotech.com> Cc: Matt Dillon <dillon@FreeBSD.org>, cvs-committers@FreeBSD.org, cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/kern vfs_bio.c Message-ID: <Pine.BSF.4.10.9909210449080.47853-100000@alphplex.bde.org> In-Reply-To: <199909201734.LAA00382@caspian.plutotech.com>
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On Mon, 20 Sep 1999, Justin T. Gibbs wrote:
> >dillon 1999/09/20 09:19:24 PDT
> >
> > Modified files:
> > sys/kern vfs_bio.c
> > Log:
> > Fix bug in brelse() regarding redirtying buffers on B_ERROR. brelse()
> > improperly ignored the B_INVAL flag when acting on the B_ERROR.
> > If both B_INVAL and B_ERROR are set the buffer is typically out of the
> > underlying device's block range and must be destroyed. If only B_ERROR
> > is set (for a write), a write error occured and operation remains as it
> > was before: the buffer must be redirtied to avoid corrupting the
> > filesystem state.
>
> If a device "goes away", how should any pending buffers be marked? Does a
> umount -f cause pending buffers to be B_INVAl'ed? I'm pretty sure that
> we still can't rid the system of the knowledge of a mounted fs for a device
> that has disappeared, but I haven't checked recently.
I think they should set B_ERROR in bp->b_flags and set bp->b_error
to something other than EIO (ENXIO perhaps), and brelse() should
only retry writes when bp->b_error == EIO. I/o beyond EOF is
already handled in this way in drivers ("something other than EIO"
is EINVAL). No device drivers set B_INVAL directly now; the above
fix is apparently for nfs.
umount -f doesn't cause buffers to be B_INVAL'ed. Writing of unwritable
buffers is retired endlessly. umount -f simply forces a write, and in
the best case, unmount(2) fails when the forced write fails. For some
filesystems, the write error is ignored and the unwritable buffer causes
a panic after unmount() blunders on. A fixed umount -f would invalidate
the buffers, perhaps after retrying a couple of times. However,
unmount() is really too late. Unwritable buffers can currently sit in
the buffer cache for as long as the system is up, causeing i/o on
every sync. They should be discarded after a few retries.
I think umount -f once handled gone-away devices as well as possible
(by forgetting about their buffers). unmount() begins by flushing
all the buffers. Unwritable buffers used to be discarded immediately
by brelse().
Bruce
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