Re: [PATCH 1/6] iomap: replace folio_batch allocation with stack allocation

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

On Thu, Oct 16, 2025 at 03:02:58PM -0400, Brian Foster wrote:
> Zhang Yi points out that the dynamic folio_batch allocation in
> iomap_fill_dirty_folios() is problematic for the ext4 on iomap work
> that is under development because it doesn't sufficiently handle the
> allocation failure case (by allowing a retry, for example).
> 
> The dynamic allocation was initially added for simplicity and to
> help indicate whether the batch was used or not by the calling fs.
> To address this issue, put the batch on the stack of
> iomap_zero_range() and use a flag to control whether the batch
> should be used in the iomap folio lookup path. This keeps things
> simple and eliminates the concern for ext4 on iomap.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>

Hrmm, so who kmallocs the fbatch array now?  Is that left as an exercise
to the filesystem?  I'm confused because I don't see the kmalloc call
reappear elsewhere, at least not in this patch.

--D

> ---
>  fs/iomap/buffered-io.c | 45 ++++++++++++++++++++++++++++--------------
>  fs/iomap/iter.c        |  6 +++---
>  fs/xfs/xfs_iomap.c     | 11 ++++++-----
>  include/linux/iomap.h  |  8 ++++++--
>  4 files changed, 45 insertions(+), 25 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 51ecb6d48feb..05ff82c5432e 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -761,7 +761,7 @@ static struct folio *__iomap_get_folio(struct iomap_iter *iter,
>  	if (!mapping_large_folio_support(iter->inode->i_mapping))
>  		len = min_t(size_t, len, PAGE_SIZE - offset_in_page(pos));
>  
> -	if (iter->fbatch) {
> +	if (iter->iomap.flags & IOMAP_F_FOLIO_BATCH) {
>  		struct folio *folio = folio_batch_next(iter->fbatch);
>  
>  		if (!folio)
> @@ -858,7 +858,7 @@ static int iomap_write_begin(struct iomap_iter *iter,
>  	 * process so return and let the caller iterate and refill the batch.
>  	 */
>  	if (!folio) {
> -		WARN_ON_ONCE(!iter->fbatch);
> +		WARN_ON_ONCE(!(iter->iomap.flags & IOMAP_F_FOLIO_BATCH));
>  		return 0;
>  	}
>  
> @@ -1473,23 +1473,34 @@ static int iomap_zero_iter(struct iomap_iter *iter, bool *did_zero,
>  	return status;
>  }
>  
> -loff_t
> +/**
> + * iomap_fill_dirty_folios - fill a folio batch with dirty folios
> + * @iter: Iteration structure
> + * @start: Start offset of range. Updated based on lookup progress.
> + * @end: End offset of range
> + *
> + * Returns the associated control flag if the folio batch is available and the
> + * lookup performed. The caller is responsible to set the flag on the associated
> + * iomap.
> + */
> +unsigned int
>  iomap_fill_dirty_folios(
>  	struct iomap_iter	*iter,
> -	loff_t			offset,
> -	loff_t			length)
> +	loff_t			*start,
> +	loff_t			end)
>  {
>  	struct address_space	*mapping = iter->inode->i_mapping;
> -	pgoff_t			start = offset >> PAGE_SHIFT;
> -	pgoff_t			end = (offset + length - 1) >> PAGE_SHIFT;
> +	pgoff_t			pstart = *start >> PAGE_SHIFT;
> +	pgoff_t			pend = (end - 1) >> PAGE_SHIFT;
>  
> -	iter->fbatch = kmalloc(sizeof(struct folio_batch), GFP_KERNEL);
> -	if (!iter->fbatch)
> -		return offset + length;
> -	folio_batch_init(iter->fbatch);
> +	if (!iter->fbatch) {
> +		*start = end;
> +		return 0;
> +	}
>  
> -	filemap_get_folios_dirty(mapping, &start, end, iter->fbatch);
> -	return (start << PAGE_SHIFT);
> +	filemap_get_folios_dirty(mapping, &pstart, pend, iter->fbatch);
> +	*start = (pstart << PAGE_SHIFT);
> +	return IOMAP_F_FOLIO_BATCH;
>  }
>  EXPORT_SYMBOL_GPL(iomap_fill_dirty_folios);
>  
> @@ -1498,17 +1509,21 @@ iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
>  		const struct iomap_ops *ops,
>  		const struct iomap_write_ops *write_ops, void *private)
>  {
> +	struct folio_batch fbatch;
>  	struct iomap_iter iter = {
>  		.inode		= inode,
>  		.pos		= pos,
>  		.len		= len,
>  		.flags		= IOMAP_ZERO,
>  		.private	= private,
> +		.fbatch		= &fbatch,
>  	};
>  	struct address_space *mapping = inode->i_mapping;
>  	int ret;
>  	bool range_dirty;
>  
> +	folio_batch_init(&fbatch);
> +
>  	/*
>  	 * To avoid an unconditional flush, check pagecache state and only flush
>  	 * if dirty and the fs returns a mapping that might convert on
> @@ -1519,11 +1534,11 @@ iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
>  	while ((ret = iomap_iter(&iter, ops)) > 0) {
>  		const struct iomap *srcmap = iomap_iter_srcmap(&iter);
>  
> -		if (WARN_ON_ONCE(iter.fbatch &&
> +		if (WARN_ON_ONCE((iter.iomap.flags & IOMAP_F_FOLIO_BATCH) &&
>  				 srcmap->type != IOMAP_UNWRITTEN))
>  			return -EIO;
>  
> -		if (!iter.fbatch &&
> +		if (!(iter.iomap.flags & IOMAP_F_FOLIO_BATCH) &&
>  		    (srcmap->type == IOMAP_HOLE ||
>  		     srcmap->type == IOMAP_UNWRITTEN)) {
>  			s64 status;
> diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
> index 66ca12aac57d..026d85823c76 100644
> --- a/fs/iomap/iter.c
> +++ b/fs/iomap/iter.c
> @@ -8,10 +8,10 @@
>  
>  static inline void iomap_iter_reset_iomap(struct iomap_iter *iter)
>  {
> -	if (iter->fbatch) {
> +	if (iter->iomap.flags & IOMAP_F_FOLIO_BATCH) {
>  		folio_batch_release(iter->fbatch);
> -		kfree(iter->fbatch);
> -		iter->fbatch = NULL;
> +		folio_batch_reinit(iter->fbatch);
> +		iter->iomap.flags &= ~IOMAP_F_FOLIO_BATCH;
>  	}
>  
>  	iter->status = 0;
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 535bf3b8705d..01833aca37ac 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -1775,7 +1775,6 @@ xfs_buffered_write_iomap_begin(
>  	 */
>  	if (flags & IOMAP_ZERO) {
>  		xfs_fileoff_t eof_fsb = XFS_B_TO_FSB(mp, XFS_ISIZE(ip));
> -		u64 end;
>  
>  		if (isnullstartblock(imap.br_startblock) &&
>  		    offset_fsb >= eof_fsb)
> @@ -1795,12 +1794,14 @@ xfs_buffered_write_iomap_begin(
>  		 */
>  		if (imap.br_state == XFS_EXT_UNWRITTEN &&
>  		    offset_fsb < eof_fsb) {
> -			loff_t len = min(count,
> -					 XFS_FSB_TO_B(mp, imap.br_blockcount));
> +			loff_t foffset = offset, fend;
>  
> -			end = iomap_fill_dirty_folios(iter, offset, len);
> +			fend = offset +
> +			       min(count, XFS_FSB_TO_B(mp, imap.br_blockcount));
> +			iomap_flags |= iomap_fill_dirty_folios(iter, &foffset,
> +							       fend);
>  			end_fsb = min_t(xfs_fileoff_t, end_fsb,
> -					XFS_B_TO_FSB(mp, end));
> +					XFS_B_TO_FSB(mp, foffset));
>  		}
>  
>  		xfs_trim_extent(&imap, offset_fsb, end_fsb - offset_fsb);
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index cd0f573156d6..79da917ff45e 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -87,6 +87,9 @@ struct vm_fault;
>  /*
>   * Flags set by the core iomap code during operations:
>   *
> + * IOMAP_F_FOLIO_BATCH indicates that the folio batch mechanism is active
> + * for this operation, set by iomap_fill_dirty_folios().
> + *
>   * IOMAP_F_SIZE_CHANGED indicates to the iomap_end method that the file size
>   * has changed as the result of this write operation.
>   *
> @@ -94,6 +97,7 @@ struct vm_fault;
>   * range it covers needs to be remapped by the high level before the operation
>   * can proceed.
>   */
> +#define IOMAP_F_FOLIO_BATCH	(1U << 13)
>  #define IOMAP_F_SIZE_CHANGED	(1U << 14)
>  #define IOMAP_F_STALE		(1U << 15)
>  
> @@ -351,8 +355,8 @@ bool iomap_dirty_folio(struct address_space *mapping, struct folio *folio);
>  int iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,
>  		const struct iomap_ops *ops,
>  		const struct iomap_write_ops *write_ops);
> -loff_t iomap_fill_dirty_folios(struct iomap_iter *iter, loff_t offset,
> -		loff_t length);
> +unsigned int iomap_fill_dirty_folios(struct iomap_iter *iter, loff_t *start,
> +		loff_t end);
>  int iomap_zero_range(struct inode *inode, loff_t pos, loff_t len,
>  		bool *did_zero, const struct iomap_ops *ops,
>  		const struct iomap_write_ops *write_ops, void *private);
> -- 
> 2.51.0
> 
>