f2fs: introduce max_io_bytes, a sysfs entry, to limit bio size

This patch adds max_io_bytes to limit bio size when f2fs tries to merge
consecutive IOs. This can give a testing point to split out bios and check
end_io handles those bios correctly. This is used to capture a recent bug
on the decompression and fsverity flow.
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

Documentation/ABI/testing/sysfs-fs-f2fs

@@ -370,3 +370,10 @@ Date:		April 2020
 Contact:	"Daeho Jeong" <daehojeong@google.com>
 Description:	Give a way to change iostat_period time. 3secs by default.
 		The new iostat trace gives stats gap given the period.
+What:		/sys/fs/f2fs/<disk>/max_io_bytes
+Date:		December 2020
+Contact:	"Jaegeuk Kim" <jaegeuk@kernel.org>
+Description:	This gives a control to limit the bio size in f2fs.
+		Default is zero, which will follow underlying block layer limit,
+		whereas, if it has a certain bytes value, f2fs won't submit a
+		bio larger than that size.

fs/f2fs/data.c

@@ -736,6 +736,9 @@ int f2fs_submit_page_bio(struct f2fs_io_info *fio)
 static bool page_is_mergeable(struct f2fs_sb_info *sbi, struct bio *bio,
 				block_t last_blkaddr, block_t cur_blkaddr)
 {
+	if (unlikely(sbi->max_io_bytes &&
+			bio->bi_iter.bi_size >= sbi->max_io_bytes))
+		return false;
 	if (last_blkaddr + 1 != cur_blkaddr)
 		return false;
 	return __same_bdev(sbi, cur_blkaddr, bio);

fs/f2fs/f2fs.h

@@ -1446,6 +1446,7 @@ struct f2fs_sb_info {
 	loff_t max_file_blocks;			/* max block index of file */
 	int dir_level;				/* directory level */
 	int readdir_ra;				/* readahead inode in readdir */
+	u64 max_io_bytes;			/* max io bytes to merge IOs */
 
 	block_t user_block_count;		/* # of user blocks */
 	block_t total_valid_block_count;	/* # of valid blocks */

fs/f2fs/sysfs.c

@@ -567,6 +567,7 @@ F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info,
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, iostat_enable, iostat_enable);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, iostat_period_ms, iostat_period_ms);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, readdir_ra, readdir_ra);
+F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, max_io_bytes, max_io_bytes);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, gc_pin_file_thresh, gc_pin_file_threshold);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_super_block, extension_list, extension_list);
 #ifdef CONFIG_F2FS_FAULT_INJECTION
@@ -651,6 +652,7 @@ static struct attribute *f2fs_attrs[] = {
 	ATTR_LIST(iostat_enable),
 	ATTR_LIST(iostat_period_ms),
 	ATTR_LIST(readdir_ra),
+	ATTR_LIST(max_io_bytes),
 	ATTR_LIST(gc_pin_file_thresh),
 	ATTR_LIST(extension_list),
 #ifdef CONFIG_F2FS_FAULT_INJECTION