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

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Message-ID: <CAGsJ_4y69s7-_UW=gaUuk3TJzwj3xRCoZhamsn0-6k65cbeR0A@mail.gmail.com>
Date: Wed, 21 Feb 2024 19:10:33 +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 6:55 PM Barry Song <21cnbao@...il.com> wrote:
>
> 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.

second thought,  you are right.  sorry for my noise.
The test was running on a platform like arm64 without HIGHMEM. thus,
kmap_local_page always returns mapped page_address of normal
zone.

but for platforms with HIGHMEM for example arm32, x86_32 , we can't use
the virtual address of the first page as the start address of two pages though
they are physically contiguous.

I will rework on this.  ideally, we should still avoid the memcpy though
two pages are within one nents :-) we are really this case for zswap
as the dst is always two pages in case the compressed data is
longer than the original data.

>
> >
> > 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