Re: [PATCH v2 12/16] iomap: add bias for async read requests

Joanne Koong
2025-09-16T19:14:17Z

On Fri, Sep 12, 2025 at 10:30 AM Joanne Koong <joannelkoong@gmail.com> wrote:
>
> On Thu, Sep 11, 2025 at 7:31 AM Christoph Hellwig <hch@infradead.org> wrote:
> >
> > > +static void __iomap_finish_folio_read(struct folio *folio, size_t off,
> > > + size_t len, int error, bool update_bitmap)
> > > {
> > > struct iomap_folio_state *ifs = folio->private;
> > > bool uptodate = !error;
> > > @@ -340,7 +340,7 @@ void iomap_finish_folio_read(struct folio *folio, size_t off, size_t len,
> > > unsigned long flags;
> > >
> > > spin_lock_irqsave(&ifs->state_lock, flags);
> > > - if (!error)
> > > + if (!error && update_bitmap)
> > > uptodate = ifs_set_range_uptodate(folio, ifs, off, len);
> >
> > This code sharing keeps confusing me a bit. I think it's technically
> > perfectly fine, but not helpful for readability. We'd solve that by
> > open coding the !update_bitmap case in iomap_read_folio_iter. Which
> > would also allow to use spin_lock_irq instead of spin_lock_irqsave there
> > as a nice little micro-optimization. If we'd then also get rid of the
> > error return from ->read_folio_range and always do asynchronous error
> > returns it would be even simpler.
> >
> > Or maybe I just need to live with the magic bitmap update, but the
> > fact that "len" sometimes is an actual length, and sometimes just a
> > counter for read_bytes_pending keeps confusing me
> >
>
> I think you're right, this is probably clearer without trying to share
> the function.
>
> I think maybe we can make this even simpler. Right now we mark the
> bitmap uptodate every time a range is read in but I think instead we
> can just do one bitmap uptodate operation for the entire folio when
> the read has completely finished. If we do this, then we can make
> "ifs->read_bytes_pending" back to an atomic_t since we don't save one
> atomic operation from doing it through a spinlock anymore (eg what
> commit f45b494e2a "iomap: protect read_bytes_pending with the
> state_lock" optimized). And then this bias thing can just become:
>
> if (ifs) {
> if (atomic_dec_and_test(&ifs->read_bytes_pending))
> folio_end_read(folio, !ret);
> *cur_folio_owned = true;
> }
>

This idea doesn't work unfortunately because reading in a range might fail.

I'll change this to open coding the !update_bitmap case with
spin_lock_irq, like Christoph suggested.

>
> Thanks,
> Joanne