Re: [PATCH 1/2] mm: zswap: free synchronous-IO writeback folios directly

[Alexandre Ghiti]

Hi Johannes,

On Tue, Jul 21, 2026 at 1:47 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> >
> On Sat, Jul 18, 2026 at 11:36:39AM +0200, Alexandre Ghiti wrote:
> > @@ -972,6 +972,30 @@ static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio)
> >  /*********************************
> >  * writeback code
> >  **********************************/
> > +static void zswap_writeback_free_folio(struct folio *folio)
> > +{
> > +     folio_lock(folio);
> > +
> > +     /* The folio was allocated off the LRU and nothing re-adds it here. */
> > +     VM_WARN_ON_ONCE_FOLIO(folio_test_lru(folio), folio);
> > +
> > +     /*
> > +      * Gate remove_mapping() on folio_test_swapcache(): a racing swapin may
> > +      * have freed the swap slot (folio_free_swap()) and dropped the folio from
> > +      * the cache, and remove_mapping() must not run on a non-swapcache folio
> > +      * (it would trip __remove_mapping()'s mapping == folio_mapping() check).
> > +      */
> > +     if (folio_test_swapcache(folio) &&
> > +         remove_mapping(swap_address_space(folio->swap), folio))
> > +             goto out;
> > +
> > +     /* Raced: the folio is now owned by the swapin; put it back on the LRU. */
> > +     folio_add_lru(folio);
> > +out:
> > +     folio_unlock(folio);
> > +     folio_put(folio);
> > +}
>
> Is this actually zswap-specific or should it be just swap code?
>
> Both this function and the dropbehind queue in the next patch seem
> like they could be reused if we wanted to make the generic swapout
> path do dropbehind as well.
>
> It looks odd in the next patch to have a generic swapcache &&
> dropbehind check and then call zswap code.
>

Yes, agreed, Yosry made the same comment!

I'll come back with a new version when I have numbers for generic swap folios.

Thanks,

Alex