linux-kernel - Re: [PATCH v5 02/12] crypto: acomp - Define new interfaces for compress/decompress batching.

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Message-ID: <Z7dnPh4tPxLO1UEo@google.com>
Date: Thu, 20 Feb 2025 17:32:46 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Herbert Xu <herbert@...dor.apana.org.au>,
	Barry Song <21cnbao@...il.com>
Cc: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	"nphamcs@...il.com" <nphamcs@...il.com>,
	"chengming.zhou@...ux.dev" <chengming.zhou@...ux.dev>,
	"usamaarif642@...il.com" <usamaarif642@...il.com>,
	"ryan.roberts@....com" <ryan.roberts@....com>,
	"21cnbao@...il.com" <21cnbao@...il.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"clabbe@...libre.com" <clabbe@...libre.com>,
	"ardb@...nel.org" <ardb@...nel.org>,
	"ebiggers@...gle.com" <ebiggers@...gle.com>,
	"surenb@...gle.com" <surenb@...gle.com>,
	"Accardi, Kristen C" <kristen.c.accardi@...el.com>,
	"Feghali, Wajdi K" <wajdi.k.feghali@...el.com>,
	"Gopal, Vinodh" <vinodh.gopal@...el.com>
Subject: Re: [PATCH v5 02/12] crypto: acomp - Define new interfaces for
 compress/decompress batching.

On Sun, Feb 16, 2025 at 01:17:59PM +0800, Herbert Xu wrote:
> On Mon, Jan 06, 2025 at 07:10:53PM -0800, Yosry Ahmed wrote:
> >
> > The main problem is memory usage. Zswap needs a PAGE_SIZE*2-sized
> > buffer for each request on each CPU. We preallocate these buffers to
> > avoid trying to allocate this much memory in the reclaim path (i.e.
> > potentially allocating two pages to reclaim one).
> 
> Actually this PAGE_SIZE * 2 thing baffles me.  Why would you
> allocate more memory than the input? The comment says that it's
> because certain hardware accelerators will disregard the output
> buffer length, but surely that's just a bug in the driver?
> 
> Which driver does this? We should fix it or remove it if it's
> writing output with no regard to the maximum length.
> 
> You should only ever need PAGE_SIZE for the output buffer, if
> the output exceeds that then just fail the compression.

I agree this should be fixed if it can be. This was discussed before
here:
https://lore.kernel.org/lkml/CAGsJ_4wuTZcGurby9h4PU2DwFaiEKB4bxuycaeyz3bPw3jSX3A@mail.gmail.com/

Barry is the one who brought up why we need PAGE_SIZE*2. Barry, could
you please chime in here?

> 
> Cheers,
> -- 
> Email: Herbert Xu <herbert@...dor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
>