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

["Darrick J. Wong" <djwong@kernel.org>]

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
> > 
> > 
>