[PATCH] f2fs: fix long latency due to discard during umount

[Sahitya Tummala]

F2FS already has a default timeout of 5 secs for discards that
can be issued during umount, but it can take more than the 5 sec
timeout if the underlying UFS device queue is already full and there
are no more available free tags to be used. In that case, submit_bio()
will wait for the already queued discard requests to complete to get
a free tag, which can potentially take way more than 5 sec.

Fix this by submitting the discard requests with REQ_NOWAIT
flags during umount. This will return -EAGAIN for UFS queue/tag full
scenario without waiting in the context of submit_bio(). The FS can
then handle these requests by retrying again within the stipulated
discard timeout period to avoid long latencies.

Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
---
 fs/f2fs/segment.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index fb3e531..a06bbac 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -1124,10 +1124,13 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
 	struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
 	struct list_head *wait_list = (dpolicy->type == DPOLICY_FSTRIM) ?
 					&(dcc->fstrim_list) : &(dcc->wait_list);
-	int flag = dpolicy->sync ? REQ_SYNC : 0;
+	int flag;
 	block_t lstart, start, len, total_len;
 	int err = 0;
 
+	flag = dpolicy->sync ? REQ_SYNC : 0;
+	flag |= dpolicy->type == DPOLICY_UMOUNT ? REQ_NOWAIT : 0;
+
 	if (dc->state != D_PREP)
 		return 0;
 
@@ -1203,6 +1206,11 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
 		bio->bi_end_io = f2fs_submit_discard_endio;
 		bio->bi_opf |= flag;
 		submit_bio(bio);
+		if ((flag & REQ_NOWAIT) && (dc->error == -EAGAIN)) {
+			dc->state = D_PREP;
+			err = dc->error;
+			break;
+		}
 
 		atomic_inc(&dcc->issued_discard);
 
@@ -1510,6 +1518,10 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
 			}
 
 			__submit_discard_cmd(sbi, dpolicy, dc, &issued);
+			if (dc->error == -EAGAIN) {
+				congestion_wait(BLK_RW_ASYNC, HZ/50);
+				__relocate_discard_cmd(dcc, dc);
+			}
 
 			if (issued >= dpolicy->max_requests)
 				break;
-- 
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