fs: direct-io: fix missing sdio->boundary

I encountered a hung task issue, but not a performance one.  I run DIO
on a device (need lba continuous, for example open channel ssd), maybe
hungtask in below case:

  DIO:						Checkpoint:
  get addr A(at boundary), merge into BIO,
  no submit because boundary missing
						flush dirty data(get addr A+1), wait IO(A+1)
						writeback timeout, because DIO(A) didn't submit
  get addr A+2 fail, because checkpoint is doing

dio_send_cur_page() may clear sdio->boundary, so prevent it from missing
a boundary.

Link: https://lkml.kernel.org/r/20210322042253.38312-1-jack.qiu@huawei.com
Fixes: b1058b98 ("direct-io: submit bio after boundary buffer is added to it")
Signed-off-by: Jack Qiu <jack.qiu@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/direct-io.c

@@ -812,6 +812,7 @@ submit_page_section(struct dio *dio, struct dio_submit *sdio, struct page *page,
 		    struct buffer_head *map_bh)
 {
 	int ret = 0;
+	int boundary = sdio->boundary;	/* dio_send_cur_page may clear it */
 
 	if (dio->op == REQ_OP_WRITE) {
 		/*
@@ -850,10 +851,10 @@ submit_page_section(struct dio *dio, struct dio_submit *sdio, struct page *page,
 	sdio->cur_page_fs_offset = sdio->block_in_file << sdio->blkbits;
 out:
 	/*
-	 * If sdio->boundary then we want to schedule the IO now to
+	 * If boundary then we want to schedule the IO now to
 	 * avoid metadata seeks.
 	 */
-	if (sdio->boundary) {
+	if (boundary) {
 		ret = dio_send_cur_page(dio, sdio, map_bh);
 		if (sdio->bio)
 			dio_bio_submit(dio, sdio);