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

["David Hildenbrand (Arm)" <david@kernel.org>]

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@ghiti.fr>
> ---

[...]

>  #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.

-- 
Cheers,

David