linux-kernel - Re: [PATCH v5 3/3] crypto: scompress: remove memcpy if sg

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Message-ID: <CAGsJ_4xCgSWz832N8+RsE=5StWwuc1zu3KpeCGxq7LdyVLY+Sg@mail.gmail.com>
Date: Wed, 21 Feb 2024 18:55:57 +1300
From: Barry Song <21cnbao@...il.com>
To: Herbert Xu <herbert@...dor.apana.org.au>, akpm@...ux-foundation.org
Cc: davem@...emloft.net, hannes@...xchg.org, linux-crypto@...r.kernel.org, 
	linux-mm@...ck.org, nphamcs@...il.com, yosryahmed@...gle.com, 
	zhouchengming@...edance.com, chriscli@...gle.com, chrisl@...nel.org, 
	ddstreet@...e.org, linux-kernel@...r.kernel.org, sjenning@...hat.com, 
	vitaly.wool@...sulko.com, Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v5 3/3] crypto: scompress: remove memcpy if sg_nents is 1

On Wed, Feb 21, 2024 at 6:35 PM Herbert Xu <herbert@...dor.apana.org.au> wrote:
>
> On Tue, Feb 20, 2024 at 07:44:14PM +1300, Barry Song wrote:
> > From: Barry Song <v-songbaohua@...o.com>
> >
> > while sg_nents is 1 which is always true for the current kernel
> > as the only user - zswap is the case, we should remove two big
> > memcpy.
> >
> > Signed-off-by: Barry Song <v-songbaohua@...o.com>
> > Tested-by: Chengming Zhou <zhouchengming@...edance.com>
> > ---
> >  crypto/scompress.c | 36 +++++++++++++++++++++++++++++-------
> >  1 file changed, 29 insertions(+), 7 deletions(-)
>
> This patch is independent of the other two.  Please split it
> out so I can apply it directly.

Ok. OTOH, patch 3/3 has no dependency with other patches. so patch
3/3 should be perfectly applicable to crypto :-)

Hi Andrew,
Would you please handle patch 1/3 and 2/3 in mm-tree given Herbert's ack on
1/3?

>
> > @@ -134,13 +135,25 @@ static int scomp_acomp_comp_decomp(struct acomp_req *req, int dir)
> >       scratch = raw_cpu_ptr(&scomp_scratch);
> >       spin_lock(&scratch->lock);
> >
> > -     scatterwalk_map_and_copy(scratch->src, req->src, 0, req->slen, 0);
> > +     if (sg_nents(req->src) == 1) {
> > +             src = kmap_local_page(sg_page(req->src)) + req->src->offset;
>
> What if the SG entry is longer than PAGE_SIZE (or indeed crosses a
> page boundary)? I think the test needs to be strengthened.

I don't understand what is the problem for a nents to cross two pages
as anyway they are contiguous in both physical and virtual addresses.
if they are not contiguous, they will be two nents.

>
> Thanks,
> --
> 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

Thanks
Barry