Re: [PATCH] iomap: add dirty page control to iomap_zero_iter

[Christoph Hellwig <hch@lst.de>]

On Sun, May 10, 2026 at 02:47:37PM +0800, Chi Zhiling wrote:
> From: Chi Zhiling <chizhiling@kylinos.cn>
> 
> This patch prepares the iomap framework for exFAT's upcoming migration to
> iomap. During testing of the exFAT iomap branch with xfstests generic/299
> on a VM with 8GB RAM and a 40GB disk, system unresponsiveness was observed.
> 
> iomap_zero_iter() lacked dirty page throttling, which could cause memory
> pressure when exFAT's valid_size mechanism triggers large-scale zeroing
> operations during writes beyond valid_size.
> 
> Align iomap_zero_iter() with iomap_write_iter() by adding
> balance_dirty_pages_ratelimited() to throttle dirty page generation during
> large zeroing operations. Also add cond_resched() to provide voluntary
> rescheduling points during long-running loops.

The balance_dirty_pages_ratelimited looks good.  Now with lazy preempt
we really should not need more cond_resched calls, though.

> 
> Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
> Cc: Namjae Jeon <linkinjeon@kernel.org>
> ---
> 
> exFAT iomap migration:
> https://lore.kernel.org/linux-fsdevel/20260507124238.7313-1-linkinjeon@kernel.org/
> 
>  fs/iomap/buffered-io.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index d7b648421a70..f6955786c8ad 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1543,6 +1543,8 @@ static int iomap_zero_iter(struct iomap_iter *iter, bool *did_zero,
>  		size_t offset;
>  		bool ret;
>  
> +		balance_dirty_pages_ratelimited(iter->inode->i_mapping);
> +
>  		bytes = min_t(u64, SIZE_MAX, bytes);
>  		status = iomap_write_begin(iter, write_ops, &folio, &offset,
>  				&bytes);
> @@ -1571,6 +1573,8 @@ static int iomap_zero_iter(struct iomap_iter *iter, bool *did_zero,
>  		if (WARN_ON_ONCE(!ret))
>  			return -EIO;
>  
> +		cond_resched();
> +
>  		status = iomap_iter_advance(iter, bytes);
>  		if (status)
>  			break;
> -- 
> 2.43.0
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