Re: [PATCH 1/2] nilfs2: add iomap operations for direct I/O reads

Hi Viacheslav,

On Sat, Jul 25, 2026 at 7:28=E2=80=AFAM Viacheslav Dubeyko wrote:
>
> Add iomap.c and iomap.h with a read-only nilfs_iomap_ops,
> wrapping the existing nilfs_bmap_lookup_contig() lookup
> to report a mapped range, a real hole, or an EOF-clamped
> range to iomap core.
>
> NILFS2 is a log-structured, copy-on-write filesystem:
> newly allocated blocks are only given a real disk address
> when the segment constructor writes them out as part of a log,
> which walks buffer_head lists directly and is not integrated
> with the generic address_space writeback path. Because of that,
> only the read-only side of the mapping is added - buffered writes,
> writeback, and mmap's ->page_mkwrite() will stay on the existing
> buffer_head based path (nilfs_get_block(), nilfs_write_begin/end(),
> nilfs_writepages(), nilfs_dirty_folio(), block_page_mkwrite()).
>
> Signed-off-by: Viacheslav Dubeyko <[email protected]>
> cc: Christoph Hellwig <[email protected]>
> cc: Ryusuke Konishi <[email protected]>
> cc: [email protected]
> cc: [email protected]
> ---
>  fs/nilfs2/Makefile |  2 +-
>  fs/nilfs2/iomap.c  | 71 ++++++++++++++++++++++++++++++++++++++++++++++
>  fs/nilfs2/iomap.h  | 13 +++++++++
>  3 files changed, 85 insertions(+), 1 deletion(-)
>  create mode 100644 fs/nilfs2/iomap.c
>  create mode 100644 fs/nilfs2/iomap.h
>
> diff --git a/fs/nilfs2/Makefile b/fs/nilfs2/Makefile
> index 43b60b8a4d07..516e6b85a03c 100644
> --- a/fs/nilfs2/Makefile
> +++ b/fs/nilfs2/Makefile
> @@ -3,4 +3,4 @@ obj-$(CONFIG_NILFS2_FS) +=3D nilfs2.o
>  nilfs2-y :=3D inode.o file.o dir.o super.o namei.o page.o mdt.o \
>         btnode.o bmap.o btree.o direct.o dat.o recovery.o \
>         the_nilfs.o segbuf.o segment.o cpfile.o sufile.o \
> -       ifile.o alloc.o gcinode.o ioctl.o sysfs.o
> +       ifile.o alloc.o gcinode.o ioctl.o sysfs.o iomap.o
> diff --git a/fs/nilfs2/iomap.c b/fs/nilfs2/iomap.c
> new file mode 100644
> index 000000000000..3ae3bf6ed368
> --- /dev/null
> +++ b/fs/nilfs2/iomap.c
> @@ -0,0 +1,71 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * NILFS iomap support implementation.
> + *
> + * Written by Viacheslav Dubeyko.
> + */
> +
> +#include <linux/iomap.h>
> +#include <linux/pagemap.h>
> +#include "nilfs.h"
> +#include "mdt.h"
> +#include "iomap.h"
> +
> +static int nilfs_iomap_begin(struct inode *inode, loff_t offset,
> +                            loff_t length, unsigned int flags,
> +                            struct iomap *iomap, struct iomap *srcmap)
> +{
> +       struct the_nilfs *nilfs =3D inode->i_sb->s_fs_info;
> +       struct nilfs_inode_info *ii =3D NILFS_I(inode);
> +       sector_t blkoff =3D offset >> inode->i_blkbits;
> +       unsigned int maxblocks;
> +       __u64 blknum =3D 0;
> +       int ret;
> +
> +       /* Completely beyond EOF. Treat as hole */
> +       if (i_size_read(inode) <=3D offset) {
> +               iomap->type =3D IOMAP_HOLE;
> +               iomap->addr =3D IOMAP_NULL_ADDR;
> +               iomap->offset =3D offset;
> +               iomap->length =3D length;
> +               return 0;
> +       }
> +
> +       /* Clamp length if the requested range goes beyond i_size */
> +       if (offset + length > i_size_read(inode)) {
> +               loff_t i_size =3D i_size_read(inode);
> +               unsigned int blocksize =3D i_blocksize(inode);
> +
> +               length =3D round_up(i_size, blocksize) - offset;
> +       }
> +
> +       maxblocks =3D min_t(loff_t, length >> inode->i_blkbits, INT_MAX);
> +       if (maxblocks =3D=3D 0)
> +               maxblocks =3D 1;
> +
> +       down_read(&NILFS_MDT(nilfs->ns_dat)->mi_sem);
> +       ret =3D nilfs_bmap_lookup_contig(ii->i_bmap, blkoff, &blknum, max=
blocks);
> +       up_read(&NILFS_MDT(nilfs->ns_dat)->mi_sem);
> +
> +       if (ret =3D=3D -ENOENT) {
> +               iomap->type =3D IOMAP_HOLE;
> +               iomap->addr =3D IOMAP_NULL_ADDR;
> +               iomap->offset =3D offset;
> +               iomap->length =3D min_t(loff_t, length, i_blocksize(inode=
));

After encountering a hole, I think we should use nilfs_bmap_seek_key()
-- as nilfs_mdt_find_block() does for metadata files -- to efficiently
determine its length.

However, if you are short on time, I am fine with the current approach as w=
ell.

Acked-by: Ryusuke Konishi <[email protected]>

Thanks,
Ryusuke Konishi

> +               return 0;
> +       } else if (ret < 0)
> +               return ret;
> +
> +       iomap->bdev =3D inode->i_sb->s_bdev;
> +       iomap->offset =3D offset;
> +       iomap->length =3D min_t(loff_t, length, (loff_t)ret << inode->i_b=
lkbits);
> +       iomap->addr =3D (loff_t)blknum << inode->i_blkbits;
> +       iomap->type =3D IOMAP_MAPPED;
> +       iomap->flags =3D IOMAP_F_MERGED;
> +
> +       return 0;
> +}
> +
> +const struct iomap_ops nilfs_iomap_ops =3D {
> +       .iomap_begin =3D nilfs_iomap_begin,
> +};
> diff --git a/fs/nilfs2/iomap.h b/fs/nilfs2/iomap.h
> new file mode 100644
> index 000000000000..adef3e22346d
> --- /dev/null
> +++ b/fs/nilfs2/iomap.h
> @@ -0,0 +1,13 @@
> +/* SPDX-License-Identifier: GPL-2.0+ */
> +/*
> + * NILFS iomap support declarations.
> + *
> + * Written by Viacheslav Dubeyko.
> + */
> +
> +#ifndef _NILFS_IOMAP_H
> +#define _NILFS_IOMAP_H
> +
> +extern const struct iomap_ops nilfs_iomap_ops;
> +
> +#endif /* _NILFS_IOMAP_H */
> --
> 2.43.0
>