Re: [RFC PATCH] mm: bypass swap readahead for zswap

[Alexandre Ghiti]

Hi Yosry,

On 6/24/26 20:01, Yosry Ahmed wrote:
> On Wed, Jun 24, 2026 at 12:57 AM Alexandre Ghiti <alex@xxxxxxxx> wrote:
> > Commit 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous
> > device") made SWP_SYNCHRONOUS_IO devices (e.g. zram) skip swap readahead.
> >
> > zswap is the same kind of in-memory, synchronous backend as zram, not a
> > swap device flagged SWP_SYNCHRONOUS_IO so it still goes through
> > swapin_readahead().
> >
> > Here are the results from bypassing readahead for zswap too: it was
> > measured with a kernel build (make -j16) in a memcg, zswap=zstd, shrinker
> > off, on Sapphire Rapids and 3 iterations.
> >
> >    768M memcg (sustained swap thrash):
> >      metric                 mm-new    + bypass    delta
> >      build time (s)          405.0       341.7    -15.6%
> >      zswap-in (GB)            79.5        53.0     -33%
> >      zswap-out (GB)          144.8       115.6     -20%
> >      swap readahead (pages)  6.79M       0.45M     -93%
> >      swap_ra hit (%)          72.1        89.9     +18pp
> >
> >    1G memcg (light pressure, build not memory-bound):
> >      metric                 mm-new    + bypass    delta
> >      build time (s)          177.7       176.0    ~same (no regression)
> >      zswap-in (GB)            10.2         7.5     -26%
> >      zswap-out (GB)           27.7        25.1      -9%
> >      swap readahead (pages)  1.07M       0.08M     -93%
> >      swap_ra hit (%)          68.6        87.2     +19pp
> >
> > The gain is from no longer prefetching pages that are pointless for an
> > in-memory backend: readahead inflates anon residency and thrashes the
> > page cache (file pages get evicted and re-read), lengthens each fault by
> > synchronously (de)compressing a cluster of neighbours, and adds
> > compression traffic when those extra pages are reclaimed.
> >
> > Bypassing swap readahead for zswap therefore makes sense.
> >
> > Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> > ---
> >
> > - This bypass originally comes from Usama's series that implements
> >    large folio zswapin: while working on improving this series, I noticed
> >    the gains I got only came from the bypass of readahead.
> >
> >   include/linux/zswap.h |  6 ++++++
> >   mm/memory.c           |  5 +++--
> >   mm/zswap.c            | 11 +++++++++++
> >   3 files changed, 20 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> > index 30c193a1207e..b6f0e6198b6f 100644
> > --- a/include/linux/zswap.h
> > +++ b/include/linux/zswap.h
> > @@ -35,6 +35,7 @@ void zswap_lruvec_state_init(struct lruvec *lruvec);
> >   void zswap_folio_swapin(struct folio *folio);
> >   bool zswap_is_enabled(void);
> >   bool zswap_never_enabled(void);
> > +bool zswap_present_test(swp_entry_t swp);
> >   #else
> >
> >   struct zswap_lruvec_state {};
> > @@ -69,6 +70,11 @@ static inline bool zswap_never_enabled(void)
> >          return true;
> >   }
> >
> > +static inline bool zswap_present_test(swp_entry_t swp)
> > +{
> > +       return false;
> > +}
> > +
> >   #endif
> >
> >   #endif /* _LINUX_ZSWAP_H */
> > diff --git a/mm/memory.c b/mm/memory.c
> > index ff338c2abe92..5aa1ea9eb48a 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4827,8 +4827,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >          if (folio)
> >                  swap_update_readahead(folio, vma, vmf->address);
> >          if (!folio) {
> > -               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices */
> > -               if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
> > +               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices and zswap */
> > +               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) ||
> > +                   zswap_present_test(entry))
> This assumes that if the swap entry is in zswap, then the remaining
> entries (covered by the readahead window) will also be in zswap,
> right? While not very likely, it's possible that the remaining entries
> not in zswap but on disk, right?


Yes, I assumed locality here.

Indeed, it would be interesting to keep the readahead but only actually 
readahead swap disk entries. I don't know how this will affect the 
readahead window (it is computed from the number of PG_readahead hits 
iirc) but I can give it a try.


> >                          folio = swapin_sync(entry, GFP_HIGHUSER_MOVABLE,
> >                                              thp_swapin_suitable_orders(vmf) | BIT(0),
> >                                              vmf, NULL, 0);
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 761cd699e0a3..5b85b4d17647 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -234,6 +234,17 @@ static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
> >                  >> ZSWAP_ADDRESS_SPACE_SHIFT];
> >   }
> >
> > +/**
> > + * zswap_present_test - check if a swap entry is currently backed by zswap
> > + * @swp: the swap entry to test
> > + *
> > + * Return: true if @swp has a zswap entry, false otherwise.
> > + */
> > +bool zswap_present_test(swp_entry_t swp)
> zswap_is_present()?


Agree, the naming is not perfect, I'll change that to either your 
proposal or zswap_is_entry_present() (or something else), but I'll 
definitely change that.

Thanks,

Alex


> > +{
> > +       return xa_load(swap_zswap_tree(swp), swp_offset(swp));
> > +}
> > +
> >   #define zswap_pool_debug(msg, p)                       \
> >          pr_debug("%s pool %s\n", msg, (p)->tfm_name)
> >
> > --
> > 2.54.0