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

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Message-ID: <B926444035E5E2439431908E3842AFD25623E4@DGGEMI525-MBS.china.huawei.com>
Date:   Thu, 9 Jul 2020 12:14:08 +0000
From:   "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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



> -----Original Message-----
> From: linux-crypto-owner@...r.kernel.org
> [mailto:linux-crypto-owner@...r.kernel.org] On Behalf Of Sebastian Andrzej
> Siewior
> Sent: Thursday, July 9, 2020 7:17 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@...ilicon.com>
> Cc: akpm@...ux-foundation.org; herbert@...dor.apana.org.au;
> davem@...emloft.net; linux-crypto@...r.kernel.org; linux-mm@...ck.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 2020-07-08 21:45:47 [+0000], Song Bao Hua (Barry Song) wrote:
> > > On 2020-07-08 00:52:10 [+1200], Barry Song wrote:
> > > > @@ -127,9 +129,17 @@
> > > > +struct crypto_acomp_ctx {
> > > > +	struct crypto_acomp *acomp;
> > > > +	struct acomp_req *req;
> > > > +	struct crypto_wait wait;
> > > > +	u8 *dstmem;
> > > > +	struct mutex mutex;
> > > > +};
> > > …
> > > > @@ -1074,12 +1138,32 @@ static int zswap_frontswap_store(unsigned
> > > type, pgoff_t offset,
> > > >  	}
> > > >
> > > >  	/* compress */
> > > > -	dst = get_cpu_var(zswap_dstmem);
> > > > -	tfm = *get_cpu_ptr(entry->pool->tfm);
> > > > -	src = kmap_atomic(page);
> > > > -	ret = crypto_comp_compress(tfm, src, PAGE_SIZE, dst, &dlen);
> > > > -	kunmap_atomic(src);
> > > > -	put_cpu_ptr(entry->pool->tfm);
> > > > +	acomp_ctx = *this_cpu_ptr(entry->pool->acomp_ctx);
> > > > +
> > > > +	mutex_lock(&acomp_ctx->mutex);
> > > > +
> > > > +	src = kmap(page);
> > > > +	dst = acomp_ctx->dstmem;
> > >
> > > that mutex is per-CPU, per-context. The dstmem pointer is per-CPU.
> > > So if I read this right, you can get preempted after
> > > crypto_wait_req() and another context in this CPU writes its data to
> > > the same dstmem and then…
> > >
> >
> > This isn't true. Another thread in this cpu will be blocked by the mutex.
> > It is impossible for two threads to write the same dstmem.
> > If thread1 ran on cpu1, it held cpu1's mutex; if another thread wants to run
> on cpu1, it is blocked.
> > If thread1 ran on cpu1 first, it held cpu1's mutex, then it migrated to cpu2
> (with very rare chance)
> > 	a. if another thread wants to run on cpu1, it is blocked;
> 
> How it is blocked? That "struct crypto_acomp_ctx" is
> "this_cpu_ptr(entry->pool->acomp_ctx)" - which is per-CPU of a pool which
> you can have multiple of. But `dstmem' you have only one per-CPU no matter
> have many pools you have.
> So pool1 on CPU1 uses the same `dstmem' as pool2 on CPU1. But pool1 and
> pool2 on CPU1 use a different mutex for protection of this `dstmem'.

Good catch, Sebastian, thanks!
this is a corner case testing has not encountered yet. There is a race if we change the pool type at runtime.
Typically, a group of initial parameters were set, then software wrote/read lots of anon pages to generate
swapping as busy as possible. But never tried to change the compressor/pool type at runtime.

will address this problem in v5 with the cleanup of acomp_ctx pointer in zswap_pool. I mean to
create acomp instants for per-cpu, not for (pools * per-cpu).

Thanks
Barry