Re: [PATCH v3 12/16] fuse: use iomap for buffered writes

Darrick J. Wong
2025-07-02T17:55:10Z

On Mon, Jun 23, 2025 at 07:21:31PM -0700, Joanne Koong wrote:
> Have buffered writes go through iomap. This has two advantages:
> * granular large folio synchronous reads
> * granular large folio dirty tracking
>
> If for example there is a 1 MB large folio and a write issued at pos 1
> to pos 1 MB - 2, only the head and tail pages will need to be read in
> and marked uptodate instead of the entire folio needing to be read in.
> Non-relevant trailing pages are also skipped (eg if for a 1 MB large
> folio a write is issued at pos 1 to 4099, only the first two pages are
> read in and the ones after that are skipped).
>
> iomap also has granular dirty tracking. This is useful in that when it
> comes to writeback time, only the dirty portions of the large folio will
> be written instead of having to write out the entire folio. For example
> if there is a 1 MB large folio and only 2 bytes in it are dirty, only
> the page for those dirty bytes get written out. Please note that
> granular writeback is only done once fuse also uses iomap in writeback
> (separate commit).
>
> .release_folio needs to be set to iomap_release_folio so that any
> allocated iomap ifs structs get freed.

What happens in the !iomap case, which can still happen for
!writeback_cache filesystems? I don't think you can call
iomap_release_folio, because iomap doesn't own folio->private in that
case.

--D

> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
> fs/fuse/Kconfig | 1 +
> fs/fuse/file.c | 140 ++++++++++++++++++------------------------------
> 2 files changed, 53 insertions(+), 88 deletions(-)
>
> diff --git a/fs/fuse/Kconfig b/fs/fuse/Kconfig
> index ca215a3cba3e..a774166264de 100644
> --- a/fs/fuse/Kconfig
> +++ b/fs/fuse/Kconfig
> @@ -2,6 +2,7 @@
> config FUSE_FS
> tristate "FUSE (Filesystem in Userspace) support"
> select FS_POSIX_ACL
> + select FS_IOMAP
> help
> With FUSE it is possible to implement a fully functional filesystem
> in a userspace program.
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index f102afc03359..a7f11c1a4f89 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -21,6 +21,7 @@
> #include <linux/filelock.h>
> #include <linux/splice.h>
> #include <linux/task_io_accounting_ops.h>
> +#include <linux/iomap.h>
>
> static int fuse_send_open(struct fuse_mount *fm, u64 nodeid,
> unsigned int open_flags, int opcode,
> @@ -788,12 +789,16 @@ static void fuse_short_read(struct inode *inode, u64 attr_ver, size_t num_read,
> }
> }
>
> -static int fuse_do_readfolio(struct file *file, struct folio *folio)
> +static int fuse_do_readfolio(struct file *file, struct folio *folio,
> + size_t off, size_t len)
> {
> struct inode *inode = folio->mapping->host;
> struct fuse_mount *fm = get_fuse_mount(inode);
> - loff_t pos = folio_pos(folio);
> - struct fuse_folio_desc desc = { .length = folio_size(folio) };
> + loff_t pos = folio_pos(folio) + off;
> + struct fuse_folio_desc desc = {
> + .offset = off,
> + .length = len,
> + };
> struct fuse_io_args ia = {
> .ap.args.page_zeroing = true,
> .ap.args.out_pages = true,
> @@ -820,8 +825,6 @@ static int fuse_do_readfolio(struct file *file, struct folio *folio)
> if (res < desc.length)
> fuse_short_read(inode, attr_ver, res, &ia.ap);
>
> - folio_mark_uptodate(folio);
> -
> return 0;
> }
>
> @@ -834,13 +837,26 @@ static int fuse_read_folio(struct file *file, struct folio *folio)
> if (fuse_is_bad(inode))
> goto out;
>
> - err = fuse_do_readfolio(file, folio);
> + err = fuse_do_readfolio(file, folio, 0, folio_size(folio));
> + if (!err)
> + folio_mark_uptodate(folio);
> +
> fuse_invalidate_atime(inode);
> out:
> folio_unlock(folio);
> return err;
> }
>
> +static int fuse_iomap_read_folio_range(const struct iomap_iter *iter,
> + struct folio *folio, loff_t pos,
> + size_t len)
> +{
> + struct file *file = iter->private;
> + size_t off = offset_in_folio(folio, pos);
> +
> + return fuse_do_readfolio(file, folio, off, len);
> +}
> +
> static void fuse_readpages_end(struct fuse_mount *fm, struct fuse_args *args,
> int err)
> {
> @@ -1375,6 +1391,24 @@ static void fuse_dio_unlock(struct kiocb *iocb, bool exclusive)
> }
> }
>
> +static const struct iomap_write_ops fuse_iomap_write_ops = {
> + .read_folio_range = fuse_iomap_read_folio_range,
> +};
> +
> +static int fuse_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
> + unsigned int flags, struct iomap *iomap,
> + struct iomap *srcmap)
> +{
> + iomap->type = IOMAP_MAPPED;
> + iomap->length = length;
> + iomap->offset = offset;
> + return 0;
> +}
> +
> +static const struct iomap_ops fuse_iomap_ops = {
> + .iomap_begin = fuse_iomap_begin,
> +};
> +
> static ssize_t fuse_cache_write_iter(struct kiocb *iocb, struct iov_iter *from)
> {
> struct file *file = iocb->ki_filp;
> @@ -1384,6 +1418,7 @@ static ssize_t fuse_cache_write_iter(struct kiocb *iocb, struct iov_iter *from)
> struct inode *inode = mapping->host;
> ssize_t err, count;
> struct fuse_conn *fc = get_fuse_conn(inode);
> + bool writeback = false;
>
> if (fc->writeback_cache) {
> /* Update size (EOF optimization) and mode (SUID clearing) */
> @@ -1397,8 +1432,7 @@ static ssize_t fuse_cache_write_iter(struct kiocb *iocb, struct iov_iter *from)
> file_inode(file))) {
> goto writethrough;
> }
> -
> - return generic_file_write_iter(iocb, from);
> + writeback = true;
> }
>
> writethrough:
> @@ -1420,6 +1454,15 @@ static ssize_t fuse_cache_write_iter(struct kiocb *iocb, struct iov_iter *from)
> goto out;
> written = direct_write_fallback(iocb, from, written,
> fuse_perform_write(iocb, from));
> + } else if (writeback) {
> + /*
> + * Use iomap so that we can do granular uptodate reads
> + * and granular dirty tracking for large folios.
> + */
> + written = iomap_file_buffered_write(iocb, from,
> + &fuse_iomap_ops,
> + &fuse_iomap_write_ops,
> + file);
> } else {
> written = fuse_perform_write(iocb, from);
> }
> @@ -2209,84 +2252,6 @@ static int fuse_writepages(struct address_space *mapping,
> return err;
> }
>
> -/*
> - * It's worthy to make sure that space is reserved on disk for the write,
> - * but how to implement it without killing performance need more thinking.
> - */
> -static int fuse_write_begin(struct file *file, struct address_space *mapping,
> - loff_t pos, unsigned len, struct folio **foliop, void **fsdata)
> -{
> - pgoff_t index = pos >> PAGE_SHIFT;
> - struct fuse_conn *fc = get_fuse_conn(file_inode(file));
> - struct folio *folio;
> - loff_t fsize;
> - int err = -ENOMEM;
> -
> - WARN_ON(!fc->writeback_cache);
> -
> - folio = __filemap_get_folio(mapping, index, FGP_WRITEBEGIN,
> - mapping_gfp_mask(mapping));
> - if (IS_ERR(folio))
> - goto error;
> -
> - if (folio_test_uptodate(folio) || len >= folio_size(folio))
> - goto success;
> - /*
> - * Check if the start of this folio comes after the end of file,
> - * in which case the readpage can be optimized away.
> - */
> - fsize = i_size_read(mapping->host);
> - if (fsize <= folio_pos(folio)) {
> - size_t off = offset_in_folio(folio, pos);
> - if (off)
> - folio_zero_segment(folio, 0, off);
> - goto success;
> - }
> - err = fuse_do_readfolio(file, folio);
> - if (err)
> - goto cleanup;
> -success:
> - *foliop = folio;
> - return 0;
> -
> -cleanup:
> - folio_unlock(folio);
> - folio_put(folio);
> -error:
> - return err;
> -}
> -
> -static int fuse_write_end(struct file *file, struct address_space *mapping,
> - loff_t pos, unsigned len, unsigned copied,
> - struct folio *folio, void *fsdata)
> -{
> - struct inode *inode = folio->mapping->host;
> -
> - /* Haven't copied anything? Skip zeroing, size extending, dirtying. */
> - if (!copied)
> - goto unlock;
> -
> - pos += copied;
> - if (!folio_test_uptodate(folio)) {
> - /* Zero any unwritten bytes at the end of the page */
> - size_t endoff = pos & ~PAGE_MASK;
> - if (endoff)
> - folio_zero_segment(folio, endoff, PAGE_SIZE);
> - folio_mark_uptodate(folio);
> - }
> -
> - if (pos > inode->i_size)
> - i_size_write(inode, pos);
> -
> - folio_mark_dirty(folio);
> -
> -unlock:
> - folio_unlock(folio);
> - folio_put(folio);
> -
> - return copied;
> -}
> -
> static int fuse_launder_folio(struct folio *folio)
> {
> int err = 0;
> @@ -3145,11 +3110,10 @@ static const struct address_space_operations fuse_file_aops = {
> .writepages = fuse_writepages,
> .launder_folio = fuse_launder_folio,
> .dirty_folio = filemap_dirty_folio,
> + .release_folio = iomap_release_folio,
> .migrate_folio = filemap_migrate_folio,
> .bmap = fuse_bmap,
> .direct_IO = fuse_direct_IO,
> - .write_begin = fuse_write_begin,
> - .write_end = fuse_write_end,
> };
>
> void fuse_init_file_inode(struct inode *inode, unsigned int flags)
> --
> 2.47.1
>
>