Re: [PATCH v4] mm/zswap: move to use crypto_acomp API for hardware acceleration

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: "Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>
Cc: "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"herbert@gondor.apana.org.au" <herbert@gondor.apana.org.au>,
	"davem@davemloft.net" <davem@davemloft.net>,
	"linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Linuxarm <linuxarm@huawei.com>,
	"Luis Claudio R . Goncalves" <lgoncalv@redhat.com>,
	Mahipal Challa <mahipalreddy2006@gmail.com>,
	Seth Jennings <sjenning@redhat.com>,
	Dan Streetman <ddstreet@ieee.org>,
	Vitaly Wool <vitaly.wool@konsulko.com>,
	"Wangzhou (B)" <wangzhou1@hisilicon.com>,
	Colin Ian King <colin.king@canonical.com>
Subject: Re: [PATCH v4] mm/zswap: move to use crypto_acomp API for hardware acceleration
Date: Thu, 9 Jul 2020 09:39:05 +0200	[thread overview]
Message-ID: <20200709073905.lgs5kvccnz6eqsyd@linutronix.de> (raw)
In-Reply-To: <B926444035E5E2439431908E3842AFD25610B7@DGGEMI525-MBS.china.huawei.com>

On 2020-07-09 01:32:38 [+0000], Song Bao Hua (Barry Song) wrote:
> > This looks using the same synchronous mechanism around an asynchronous
> > interface. It works as a PoC.
> > 
> > As far as I remember the crypto async interface, the incoming skbs were fed to
> > the async interface and returned to the caller so the NIC could continue
> > allocate new RX skbs and move on. Only if the queue of requests was getting
> > to long the code started to throttle. Eventually the async crypto code
> > completed the decryption operation in a different context and fed the
> > decrypted packet(s) into the stack.
> > 
> > From a quick view, you would have to return -EINPROGRESS here and have at
> > the caller side something like that:
> > 
> > iff --git a/mm/page_io.c b/mm/page_io.c
> > index e8726f3e3820b..9d1baa46ec3ed 100644
> > --- a/mm/page_io.c
> > +++ b/mm/page_io.c
> > @@ -252,12 +252,15 @@ int swap_writepage(struct page *page, struct
> > writeback_control *wbc)
> >                 unlock_page(page);
> >                 goto out;
> >         }
> > -       if (frontswap_store(page) == 0) {
> > +       ret = frontswap_store(page);
> > +       if (ret == 0) {
> >                 set_page_writeback(page);
> >                 unlock_page(page);
> >                 end_page_writeback(page);
> >                 goto out;
> >         }
> > +       if (ret = -EINPROGRESS)
> > +               goto out;
> >         ret = __swap_writepage(page, wbc, end_swap_bio_write);
> >  out:
> >         return ret;
> > 
> Unfortunately, this is not true and things are not that simple.
> 
> We can't simply depend on -EINPROGRESS and go out.
> We have to wait for the result of compression to decide if we should
> do __swap_writepage(). As one page might be compressed into two
> pages, in this case, we will still need to do _swap_writepage().
> As I replied in the latest email, all of the async improvement to frontswap
> needs very careful thinking and benchmark. It can only happen after
> we build the base in this patch, fixing the broken connection between
> zswap and those new zip drivers.

At the time the compression finishes you see what happens and based on
the design you can either complete it immediately (the 0/error case from
above) or forward the result to the caller and let him decide.

> Thanks
> Barry

Sebastian