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

Darrick J. Wong
2025-07-02T17:57:44Z

On Wed, Jul 02, 2025 at 10:55:09AM -0700, Darrick J. Wong wrote:
> 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.

...and I think the answer to that is that the !writeback_cache case
passes all file IO directly to the fuse server and never touches the
page cache at all?

--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
> >
> >
>