Re: [PATCH v9 6/7] mm: zswap: Support large folios in zswap_store().

[Nhat Pham <nphamcs@gmail.com>]

On Mon, Sep 30, 2024 at 4:34 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Mon, Sep 30, 2024 at 4:29 PM Nhat Pham <nphamcs@gmail.com> wrote:
> >
> > On Mon, Sep 30, 2024 at 4:20 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> > >
> > > On Mon, Sep 30, 2024 at 4:11 PM Nhat Pham <nphamcs@gmail.com> wrote:
> > >
> > > I suggested this in a previous version, and Kanchana faced some
> > > complexities implementing it:
> > > https://lore.kernel.org/lkml/SJ0PR11MB56785027ED6FCF673A84CEE6C96A2@SJ0PR11MB5678.namprd11.prod.outlook.com/
> >
> > Sorry, I missed that conversation.
> >
> > >
> > > Basically, if we batch get the refs after the store I think it's not
> > > safe, because once an entry is published to writeback it can be
> > > written back and freed, and a ref that we never acquired would be
> > > dropped.
> >
> > Hmmm. I don't think writeback could touch any individual subpage just yet, no?
> >
> > Before doing any work, zswap writeback would attempt to add the
> > subpage to the swap cache (via __read_swap_cache_async()). However,
> > all subpage will have already been added to swap cache, and point to
> > the (large) folio. So zswap_writeback_entry() should short circuit
> > here (the if (!page_allocated) case).
>
> If it's safe to take the refs after all calls to zswap_store_page()
> are successful, then yeah that should be possible, for both the pool
> and objcg. I didn't look closely though.
>
> Just to clarify, you mean grab one ref first, then do the
> compressions, then grab the remaining refs, right?

Ah yeah, that's what I meant. We can either perform one of the
following sequences: grab 1 -> grab nr -> drop 1, or grab 1 -> grab nr
- 1 if successful, drop 1 if failed.

Seems straightforward to me, but yeah it seems a bit hair-splitting of
me to die on this hill :) Just thought it was weird seeing the other
parts batchified, and one part wasn't.

The rest LGTM - I'll defer to you and Johannes for further review.

Reviewed-by: Nhat Pham <nphamcs@gmail.com>