lightnvm: pblk: guarantee that backpointer is respected on writer stall

pblk's write buffer must guarantee that it respects the device's
constrains for reads (i.e., mw_cunits). This is done by maintaining a
backpointer that updates the L2P table as entries wrap up, making them
point to the media instead of pointing to the write buffer.

This mechanism can race in case that the write thread stalls, as the
write pointer will protect the last written entry, thus disregarding the
read constrains.

This patch adds an extra check on wrap up, making sure that the
threshold is respected at all times, preventing new entries to overwrite
committed data, also in case of write thread stall.

Reported-by: Heiner Litz <hlitz@ucsc.edu>
Signed-off-by: Javier González <javier@cnexlabs.com>
Reviewed-by: Heiner Litz <hlitz@ucsc.edu>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 7 insertions, 1 deletions

diff --git a/drivers/lightnvm/pblk.h b/drivers/lightnvm/pblk.h
index 0f98ea24ee59..02bb2e98f8a9 100644
--- a/drivers/lightnvm/pblk.h
+++ b/drivers/lightnvm/pblk.h
@@ -203,6 +203,11 @@ struct pblk_rb {
                                          * will be 4KB
                                          */
 
+        unsigned int back_thres;        /* Threshold that shall be maintained by
+                                         * the backpointer in order to respect
+                                         * geo->mw_cunits on a per chunk basis
+                                         */
+
         struct list_head pages;         /* List of data pages */
 
         spinlock_t w_lock;              /* Write lock */
@@ -734,7 +739,8 @@ struct pblk_line_ws {
 /*
  * pblk ring buffer operations
  */
-int pblk_rb_init(struct pblk_rb *rb, unsigned int size, unsigned int seg_sz);
+int pblk_rb_init(struct pblk_rb *rb, unsigned int size, unsigned int threshold,
+                 unsigned int seg_sz);
 int pblk_rb_may_write_user(struct pblk_rb *rb, struct bio *bio,
                            unsigned int nr_entries, unsigned int *pos);
 int pblk_rb_may_write_gc(struct pblk_rb *rb, unsigned int nr_entries,