Fix failure to verify in mixed read/write workload with backlog

If you run a workload like this:

fio --rw=randrw --bs=4k --direct=1 --ioengine=libaio --iodepth=32
--verify=meta --verify_backlog=1024 --verify_fatal=1 --name=ver-test
--filename=foo --size=1G --verify_pattern=0xaaa

Fio ends up never actually verifying the written blocks. This happens
because as we generate an entry to be verified, the backend checks
whether this is a read/write mixed workload. It then thinks that the
READ is just that, a normal READ, when in fact it could be coming
from our verify list.

Mark such a read as coming from our verify list, so that we know if
it's a "normal" read or one generated to verify previously written
data.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/backend.c b/backend.c
index 814d244..7343286 100644
--- a/backend.c
+++ b/backend.c
@@ -586,11 +586,12 @@
 		ddir = io_u->ddir;
 
 		/*
-		 * Add verification end_io handler, if asked to verify
-		 * a previously written file.
+		 * Add verification end_io handler if:
+		 *	- Asked to verify (!td_rw(td))
+		 *	- Or the io_u is from our verify list (mixed write/ver)
 		 */
 		if (td->o.verify != VERIFY_NONE && io_u->ddir == DDIR_READ &&
-		    !td_rw(td)) {
+		    ((io_u->flags & IO_U_F_VER_LIST) || !td_rw(td))) {
 			if (td->o.verify_async)
 				io_u->end_io = verify_io_u_async;
 			else
diff --git a/io_u.c b/io_u.c
index a0020d2..20794c3 100644
--- a/io_u.c
+++ b/io_u.c
@@ -1070,6 +1070,7 @@
 		assert(io_u->flags & IO_U_F_FREE);
 		io_u->flags &= ~(IO_U_F_FREE | IO_U_F_FREE_DEF);
 		io_u->flags &= ~(IO_U_F_TRIMMED | IO_U_F_BARRIER);
+		io_u->flags &= ~IO_U_F_VER_LIST;
 
 		io_u->error = 0;
 		flist_del(&io_u->list);
diff --git a/ioengine.h b/ioengine.h
index efca45e..61cb396 100644
--- a/ioengine.h
+++ b/ioengine.h
@@ -11,6 +11,7 @@
 	IO_U_F_BUSY_OK		= 1 << 4,
 	IO_U_F_TRIMMED		= 1 << 5,
 	IO_U_F_BARRIER		= 1 << 6,
+	IO_U_F_VER_LIST		= 1 << 7,
 };
 
 /*
diff --git a/verify.c b/verify.c
index 6dd7f6a..f25eab9 100644
--- a/verify.c
+++ b/verify.c
@@ -973,6 +973,7 @@
 		io_u->offset = ipo->offset;
 		io_u->buflen = ipo->len;
 		io_u->file = ipo->file;
+		io_u->flags |= IO_U_F_VER_LIST;
 
 		if (ipo->flags & IP_F_TRIMMED)
 			io_u->flags |= IO_U_F_TRIMMED;