Re: [PATCH v2] iomap: move prefaulting out of hot write path

[Christoph Hellwig]

On Thu, Jul 31, 2025 at 12:44:09AM +0800, alexjlzheng@xxxxxxxxx wrote:
> From: Jinliang Zheng <alexjlzheng@xxxxxxxxxxx>
> 
> Prefaulting the write source buffer incurs an extra userspace access
> in the common fast path. Make iomap_write_iter() consistent with
> generic_perform_write(): only touch userspace an extra time when
> copy_folio_from_iter_atomic() has failed to make progress.

This is probably a good thing to have, but I'm curous if you did see
it making a different for workloads?

> +		/*
> +		 * Faults here on mmap()s can recurse into arbitrary
> +		 * filesystem code. Lots of locks are held that can
> +		 * deadlock. Use an atomic copy to avoid deadlocking
> +		 * in page fault handling.

We can and should use all 80 characters in a line for comments.

> +			/*
> +			 * 'folio' is now unlocked and faults on it can be
> +			 * handled. Ensure forward progress by trying to
> +			 * fault it in now.
> +			 */

Same here.

I really wish we could find a way to share the core write loop between
at least iomap and generic_perform_write and maybe also the other copy
and pasters.  But that's for another time..