[PATCH] NFS/localio: fix nfs_local_dio_misaligned tracepoint

The intended focus of nfs_local_iters_setup_dio()'s call to
trace_nfs_local_dio_misaligned() is on the middle segment being
misaligned, yet the @offset passed in was local_dio->start_len.
It would appear this was a cut-n-paste bug from the preceding
nfs_local_iter_setup() call that passes local_dio->start_len.

Fix this by passing the @offset as local_dio->middle_offset and
calculate the start segment's offset rather than assume.

Example traces, before this fix:

   python3-32744   [006] .l... 132946.352360:
   nfs_local_dio_write: fileid=00:33:1286 fhandle=0xf1f7c10b
   offset=1048759 count=1048576 mem_align=4 offset_align=512
   start=1048759+329 middle=1049088+1048064 end=2097152+183

   python3-32744   [006] .l... 132946.352360:
   nfs_local_dio_misaligned: fileid=00:33:1286 fhandle=0xf1f7c10b
   offset=329 count=1048064 mem_align=4 offset_align=512
   start=329+329 middle=1049088+1048064 end=2097152+183

After this fix:

   python3-32744   [006] .l... 132946.352360:
   nfs_local_dio_write: fileid=00:33:1286 fhandle=0xf1f7c10b
   offset=1048759 count=1048576 mem_align=4 offset_align=512
   start=1048759+329 middle=1049088+1048064 end=2097152+183

   python3-32744   [006] .l... 132946.352360:
   nfs_local_dio_misaligned: fileid=00:33:1286 fhandle=0xf1f7c10b
   offset=1049088 count=1048064 mem_align=4 offset_align=512
   start=1048759+329 middle=1049088+1048064 end=2097152+183

Cc: [email protected]
Signed-off-by: Mike Snitzer <[email protected]>
---
 fs/nfs/localio.c  | 2 +-
 fs/nfs/nfstrace.h | 5 ++++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/nfs/localio.c b/fs/nfs/localio.c
index f42b6112a613..63c38dea50cc 100644
--- a/fs/nfs/localio.c
+++ b/fs/nfs/localio.c
@@ -446,7 +446,7 @@ nfs_local_iters_setup_dio(struct nfs_local_kiocb *iocb, int rw,
 
 	if (unlikely(!iocb->iter_is_dio_aligned[n_iters])) {
 		trace_nfs_local_dio_misaligned(iocb->hdr->inode,
-			local_dio->start_len, local_dio->middle_len, local_dio);
+			local_dio->middle_offset, local_dio->middle_len, local_dio);
 		return 0; /* no DIO-aligned IO possible */
 	}
 	iocb->end_iter_index = n_iters;
diff --git a/fs/nfs/nfstrace.h b/fs/nfs/nfstrace.h
index c70debb88aa1..2e1932962a81 100644
--- a/fs/nfs/nfstrace.h
+++ b/fs/nfs/nfstrace.h
@@ -1772,7 +1772,10 @@ DECLARE_EVENT_CLASS(nfs_local_dio_class,
 		__entry->count = count;
 		__entry->mem_align = local_dio->mem_align;
 		__entry->offset_align = local_dio->offset_align;
-		__entry->start = offset;
+		if (local_dio->start_len)
+			__entry->start = local_dio->middle_offset - local_dio->start_len;
+		else
+			__entry->start = 0;
 		__entry->start_len = local_dio->start_len;
 		__entry->middle = local_dio->middle_offset;
 		__entry->middle_len = local_dio->middle_len;
-- 
2.43.0