lightnvm: pblk: enable 1 LUN configuration

Metadata I/Os are scheduled to minimize their impact on user data I/Os.
When there are enough LUNs instantiated (i.e., enough bandwidth), it is
easy to interleave metadata and data one after the other so that
metadata I/Os are the ones being blocked and not vice-versa.

We do this by calculating the distance between the I/Os in terms of the
LUNs that are not in used, and selecting a free LUN that satisfies a
the simple heuristic that metadata is scheduled behind. The per-LUN
semaphores guarantee consistency. This works fine on >1 LUN
configuration. However, when a single LUN is instantiated, this design
leads to a deadlock, where metadata waits to be scheduled on a free LUN.

This patch implements the 1 LUN case by simply scheduling the metadada
I/O after the data I/O. In the process, we refactor the way a line is
replaced to ensure that metadata writes are submitted after data writes
in order to guarantee block sequentiality. Note that, since there is
only one LUN, both I/Os will block each other by design. However, such
configuration only pursues tight read latencies, not write bandwidth.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 12 insertions, 9 deletions

diff --git a/drivers/lightnvm/pblk-map.c b/drivers/lightnvm/pblk-map.c
index fddb924f6dde..3bc4c94f9cf2 100644
--- a/drivers/lightnvm/pblk-map.c
+++ b/drivers/lightnvm/pblk-map.c
@@ -25,13 +25,23 @@ static void pblk_map_page_data(struct pblk *pblk, unsigned int sentry,
                                unsigned int valid_secs)
 {
         struct pblk_line *line = pblk_line_get_data(pblk);
-        struct pblk_emeta *emeta = line->emeta;
+        struct pblk_emeta *emeta;
         struct pblk_w_ctx *w_ctx;
-        __le64 *lba_list = emeta_to_lbas(pblk, emeta->buf);
+        __le64 *lba_list;
         u64 paddr;
         int nr_secs = pblk->min_write_pgs;
         int i;
 
+        if (pblk_line_is_full(line)) {
+                struct pblk_line *prev_line = line;
+
+                line = pblk_line_replace_data(pblk);
+                pblk_line_close_meta(pblk, prev_line);
+        }
+
+        emeta = line->emeta;
+        lba_list = emeta_to_lbas(pblk, emeta->buf);
+
         paddr = pblk_alloc_page(pblk, line, nr_secs);
 
         for (i = 0; i < nr_secs; i++, paddr++) {
@@ -60,13 +70,6 @@ static void pblk_map_page_data(struct pblk *pblk, unsigned int sentry,
                 }
         }
 
-        if (pblk_line_is_full(line)) {
-                struct pblk_line *prev_line = line;
-
-                pblk_line_replace_data(pblk);
-                pblk_line_close_meta(pblk, prev_line);
-        }
-
         pblk_down_rq(pblk, ppa_list, nr_secs, lun_bitmap);
 }