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

[Alexandre Ghiti]

Hi David,

On 6/24/26 16:58, David Hildenbrand (Arm) wrote:
> On 6/24/26 09:55, Alexandre Ghiti 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>
> > ---
> [...]
>
> >   #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 should really be abstracted into a reasonably-named helper that can live in
> swap code.

Makes sense, I'll come up with something.

Thanks,

Alex