linux-kernel - Re: [PATCH v4] mm/zswap: move to use crypto

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Date:   Thu, 9 Jul 2020 09:39:05 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
Cc:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linuxarm <linuxarm@...wei.com>,
        "Luis Claudio R . Goncalves" <lgoncalv@...hat.com>,
        Mahipal Challa <mahipalreddy2006@...il.com>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>,
        "Wangzhou (B)" <wangzhou1@...ilicon.com>,
        Colin Ian King <colin.king@...onical.com>
Subject: Re: [PATCH v4] mm/zswap: move to use crypto_acomp API for hardware
 acceleration

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