erofs: use buffered I/O for file-backed mounts by default

For many use cases (e.g. container images are just fetched from remote),
performance will be impacted if underlay page cache is up-to-date but
direct i/o flushes dirty pages first.

Instead, let's use buffered I/O by default to keep in sync with loop
devices and add a (re)mount option to explicitly give a try to use
direct I/O if supported by the underlying files.

The container startup time is improved as below:
[workload] docker.io/library/workpress:latest
                                     unpack        1st run  non-1st runs
EROFS snapshotter buffered I/O file  4.586404265s  0.308s   0.198s
EROFS snapshotter direct I/O file    4.581742849s  2.238s   0.222s
EROFS snapshotter loop               4.596023152s  0.346s   0.201s
Overlayfs snapshotter                5.382851037s  0.206s   0.214s

Fixes: fb17675026 ("erofs: add file-backed mount support")
Cc: Derek McGowan <derek@mcg.dev>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20241212134336.2059899-1-hsiangkao@linux.alibaba.com

fs/erofs/fileio.c

@@ -9,6 +9,7 @@ struct erofs_fileio_rq {
 	struct bio_vec bvecs[BIO_MAX_VECS];
 	struct bio bio;
 	struct kiocb iocb;
+	struct super_block *sb;
 };
 
 struct erofs_fileio {
@@ -52,8 +53,9 @@ static void erofs_fileio_rq_submit(struct erofs_fileio_rq *rq)
 	rq->iocb.ki_pos = rq->bio.bi_iter.bi_sector << SECTOR_SHIFT;
 	rq->iocb.ki_ioprio = get_current_ioprio();
 	rq->iocb.ki_complete = erofs_fileio_ki_complete;
-	rq->iocb.ki_flags = (rq->iocb.ki_filp->f_mode & FMODE_CAN_ODIRECT) ?
-				IOCB_DIRECT : 0;
+	if (test_opt(&EROFS_SB(rq->sb)->opt, DIRECT_IO) &&
+	    rq->iocb.ki_filp->f_mode & FMODE_CAN_ODIRECT)
+		rq->iocb.ki_flags = IOCB_DIRECT;
 	iov_iter_bvec(&iter, ITER_DEST, rq->bvecs, rq->bio.bi_vcnt,
 		      rq->bio.bi_iter.bi_size);
 	ret = vfs_iocb_iter_read(rq->iocb.ki_filp, &rq->iocb, &iter);
@@ -68,6 +70,7 @@ static struct erofs_fileio_rq *erofs_fileio_rq_alloc(struct erofs_map_dev *mdev)
 
 	bio_init(&rq->bio, NULL, rq->bvecs, BIO_MAX_VECS, REQ_OP_READ);
 	rq->iocb.ki_filp = mdev->m_dif->file;
+	rq->sb = mdev->m_sb;
 	return rq;
 }
 

fs/erofs/internal.h

@@ -176,6 +176,7 @@ struct erofs_sb_info {
 #define EROFS_MOUNT_POSIX_ACL		0x00000020
 #define EROFS_MOUNT_DAX_ALWAYS		0x00000040
 #define EROFS_MOUNT_DAX_NEVER		0x00000080
+#define EROFS_MOUNT_DIRECT_IO		0x00000100
 
 #define clear_opt(opt, option)	((opt)->mount_opt &= ~EROFS_MOUNT_##option)
 #define set_opt(opt, option)	((opt)->mount_opt |= EROFS_MOUNT_##option)

fs/erofs/super.c

@@ -364,14 +364,8 @@ static void erofs_default_options(struct erofs_sb_info *sbi)
 }
 
 enum {
-	Opt_user_xattr,
-	Opt_acl,
-	Opt_cache_strategy,
-	Opt_dax,
-	Opt_dax_enum,
-	Opt_device,
-	Opt_fsid,
-	Opt_domain_id,
+	Opt_user_xattr, Opt_acl, Opt_cache_strategy, Opt_dax, Opt_dax_enum,
+	Opt_device, Opt_fsid, Opt_domain_id, Opt_directio,
 	Opt_err
 };
 
@@ -398,6 +392,7 @@ static const struct fs_parameter_spec erofs_fs_parameters[] = {
 	fsparam_string("device",	Opt_device),
 	fsparam_string("fsid",		Opt_fsid),
 	fsparam_string("domain_id",	Opt_domain_id),
+	fsparam_flag_no("directio",	Opt_directio),
 	{}
 };
 
@@ -511,6 +506,16 @@ static int erofs_fc_parse_param(struct fs_context *fc,
 		errorfc(fc, "%s option not supported", erofs_fs_parameters[opt].name);
 		break;
 #endif
+	case Opt_directio:
+#ifdef CONFIG_EROFS_FS_BACKED_BY_FILE
+		if (result.boolean)
+			set_opt(&sbi->opt, DIRECT_IO);
+		else
+			clear_opt(&sbi->opt, DIRECT_IO);
+#else
+		errorfc(fc, "%s option not supported", erofs_fs_parameters[opt].name);
+#endif
+		break;
 	default:
 		return -ENOPARAM;
 	}
@@ -948,6 +953,8 @@ static int erofs_show_options(struct seq_file *seq, struct dentry *root)
 		seq_puts(seq, ",dax=always");
 	if (test_opt(opt, DAX_NEVER))
 		seq_puts(seq, ",dax=never");
+	if (erofs_is_fileio_mode(sbi) && test_opt(opt, DIRECT_IO))
+		seq_puts(seq, ",directio");
 #ifdef CONFIG_EROFS_FS_ONDEMAND
 	if (sbi->fsid)
 		seq_printf(seq, ",fsid=%s", sbi->fsid);