Currently, XFS uses BH_PrivateStart for flagging unwritten
extent state in a bufferhead. Recently, i found the long standing
mmap/unwritten extent conversion bug, and it was to do with
partial page invalidation not clearing the unwritten flag from
bufferheads attached to the page but beyond EOF. See here
for a full explaination:
http://oss.sgi.com/archives/xfs/2006-12/msg00196.html
The solution I have checked into the XFS dev tree involves
duplicating code from block_invalidatepage to clear the
unwritten flag from the bufferhead(s), and then calling
block_invalidatepage() to do the rest.
Christoph suggested that this would be better solved by
pushing the unwritten flag into the common buffer head flags
and just adding the call to discard_buffer():
http://oss.sgi.com/archives/xfs/2006-12/msg00239.html
The following patch makes BH_Unwritten a first class citizen.
Patch against 2.6.20-rc3.
Signed-Off-By: Dave Chinner <[email protected]>
---
fs/buffer.c | 1 +
fs/xfs/linux-2.6/xfs_linux.h | 10 ----------
include/linux/buffer_head.h | 2 ++
3 files changed, 3 insertions(+), 10 deletions(-)
Index: linux/fs/buffer.c
===================================================================
--- linux.orig/fs/buffer.c 2007-01-08 17:19:49.039465038 +1100
+++ linux/fs/buffer.c 2007-01-08 17:20:43.547898480 +1100
@@ -1439,6 +1439,7 @@ static void discard_buffer(struct buffer
clear_buffer_req(bh);
clear_buffer_new(bh);
clear_buffer_delay(bh);
+ clear_buffer_unwritten(bh);
unlock_buffer(bh);
}
Index: linux/fs/xfs/linux-2.6/xfs_linux.h
===================================================================
--- linux.orig/fs/xfs/linux-2.6/xfs_linux.h 2007-01-08 17:19:32.703335135 +1100
+++ linux/fs/xfs/linux-2.6/xfs_linux.h 2007-01-08 17:19:36.271363508 +1100
@@ -109,16 +109,6 @@
#undef HAVE_PERCPU_SB /* per cpu superblock counters are a 2.6 feature */
#endif
-/*
- * State flag for unwritten extent buffers.
- *
- * We need to be able to distinguish between these and delayed
- * allocate buffers within XFS. The generic IO path code does
- * not need to distinguish - we use the BH_Delay flag for both
- * delalloc and these ondisk-uninitialised buffers.
- */
-BUFFER_FNS(PrivateStart, unwritten);
-
#define restricted_chown xfs_params.restrict_chown.val
#define irix_sgid_inherit xfs_params.sgid_inherit.val
#define irix_symlink_mode xfs_params.symlink_mode.val
Index: linux/include/linux/buffer_head.h
===================================================================
--- linux.orig/include/linux/buffer_head.h 2007-01-08 17:17:15.118241081 +1100
+++ linux/include/linux/buffer_head.h 2007-01-08 17:18:25.714802453 +1100
@@ -34,6 +34,7 @@ enum bh_state_bits {
BH_Write_EIO, /* I/O error on write */
BH_Ordered, /* ordered write */
BH_Eopnotsupp, /* operation not supported (barrier) */
+ BH_Unwritten, /* Buffer is allocated on disk but not written */
BH_PrivateStart,/* not a state bit, but the first bit available
* for private allocation by other entities
@@ -126,6 +127,7 @@ BUFFER_FNS(Boundary, boundary)
BUFFER_FNS(Write_EIO, write_io_error)
BUFFER_FNS(Ordered, ordered)
BUFFER_FNS(Eopnotsupp, eopnotsupp)
+BUFFER_FNS(Unwritten, unwritten)
#define bh_offset(bh) ((unsigned long)(bh)->b_data & ~PAGE_MASK)
#define touch_buffer(bh) mark_page_accessed(bh->b_page)
this doesn't look like a full first class flag to me yet. Don't
we need to check for buffer_unwritten in the places we're checking
for buffer_delay so we can stop setting buffer_delay for unwritten
buffers?
On Mon, 2007-01-08 at 22:54 +0000, Christoph Hellwig wrote:
> this doesn't look like a full first class flag to me yet. Don't
> we need to check for buffer_unwritten in the places we're checking
> for buffer_delay so we can stop setting buffer_delay for unwritten
> buffers?
Yep, that does need to be done. The first of the two calls
to set_buffer_delay can be removed from __xfs_get_blocks also
(currently there is an implied association between Delay and
Unwritten, which should be removed now).
I have a vague memory of some magic sysrq code (from 2.4 days)
which counted BH state on a page - if that still exists it'd
need to be updated too, but I can't seem to find it in current
2.6 kernels (used to live in buffer.c in ye olde 2.4 days). It
probably left us around the time of PG_private's introduction.
cheers.
--
Nathan
On Mon, Jan 08, 2007 at 10:54:02PM +0000, Christoph Hellwig wrote:
> this doesn't look like a full first class flag to me yet. Don't
> we need to check for buffer_unwritten in the places we're checking
> for buffer_delay so we can stop setting buffer_delay for unwritten
> buffers?
That would be __block_prepare_write() and block_truncate_page()?
I can't see anywhere else in the code where buffer_delay is used
outside XFS....
Cheers,
Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group
On Tue, Jan 09, 2007 at 10:57:45AM +1100, David Chinner wrote:
> On Mon, Jan 08, 2007 at 10:54:02PM +0000, Christoph Hellwig wrote:
> > this doesn't look like a full first class flag to me yet. Don't
> > we need to check for buffer_unwritten in the places we're checking
> > for buffer_delay so we can stop setting buffer_delay for unwritten
> > buffers?
>
> That would be __block_prepare_write() and block_truncate_page()?
> I can't see anywhere else in the code where buffer_delay is used
> outside XFS....
Yes, I think it's just those two.