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

[Jinliang Zheng]

On Thu, 31 Jul 2025 07:21:57 -0700, Christoph Hellwig wrote:
> On Thu, Jul 32, 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?

Yes, there is some improvement. However, I tested it only a few times,
so I can't rule out jitter.

However, from a design pattern perspective, this patch is a good thing
anyway.

> 
> > +		/*
> > +		 * 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.

I agree. hahaha :)

thanks,
Jinliang Zheng. :) :)

> 
> > +			/*
> > +			 * '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..