RE: [PATCH v13 22/22] mm: zswap: Batched zswap_compress() with compress batching of large folios.

[Sridhar, Kanchana P]

> -----Original Message-----
> From: Yosry Ahmed <yosry.ahmed@xxxxxxxxx>
> Sent: Tuesday, December 9, 2025 8:55 AM
> To: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> Cc: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>; SeongJae Park
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> Subject: Re: [PATCH v13 22/22] mm: zswap: Batched zswap_compress() with
> compress batching of large folios.
> 
> On Tue, Dec 09, 2025 at 10:32:20AM +0800, Herbert Xu wrote:
> > On Tue, Dec 09, 2025 at 01:15:02AM +0000, Yosry Ahmed wrote:
> > >
> > > Just to clarify, does this mean that zswap can pass a batch of (eight)
> > > pages to the acomp API, and get the results for the batch uniformly
> > > whether or not the underlying compressor supports batching?
> >
> > Correct.  In fact I'd like to remove the batch size exposure to zswap
> > altogether.  zswap should just pass along whatever maximum number of
> > pages that is convenient to itself.
> 
> I think exposing the batch size is still useful as a hint for zswap. In
> the current series, zswap allocates as many per-CPU buffers as the
> compressor's batch size, so no extra buffers for non-batching
> compressors (including SW compressors).
> 
> If we use the same batch size regardless, we'll have to always allocate
> 8 (or N) per-CPU buffers, for little to no benefit on non-batching
> compressors.
> 
> So we still want the batch size on the zswap side, but we want the
> crypto API to be uniform whether or not the compressor supports
> batching.

Thanks Yosry, you bring up a good point. I currently have the outer for
loop in zswap_compress() due to the above constraint. For non-batching
compressors, we allocate only one per-CPU buffer. Hence, we need to
call crypto_acomp_compress() and write the compressed data to the
zs_poll for each page in the batch. Wouldn't we need to allocate
8 per-CPU buffers for non-batching compressors if we want zswap to
send a batch of 8 pages uniformly to the crypto API, so that
zswap_compress() can store the 8 pages in zs_pool after the crypto
API returns?

Thanks,
Kanchana

> 
> >
> > Cheers,
> > --
> > Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> > Home Page: http://gondor.apana.org.au/~herbert/
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