RE: [PATCH] zsmalloc: do not use bit_spin_lock

["Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>]

> -----Original Message-----
> From: Vitaly Wool [mailto:vitaly.wool@konsulko.com]
> Sent: Tuesday, December 22, 2020 2:00 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
> Cc: Shakeel Butt <shakeelb@google.com>; Minchan Kim <minchan@kernel.org>; Mike
> Galbraith <efault@gmx.de>; LKML <linux-kernel@vger.kernel.org>; linux-mm
> <linux-mm@kvack.org>; Sebastian Andrzej Siewior <bigeasy@linutronix.de>;
> NitinGupta <ngupta@vflare.org>; Sergey Senozhatsky
> <sergey.senozhatsky.work@gmail.com>; Andrew Morton
> <akpm@linux-foundation.org>
> Subject: Re: [PATCH] zsmalloc: do not use bit_spin_lock
> 
> On Tue, Dec 22, 2020 at 12:37 AM Song Bao Hua (Barry Song)
> <song.bao.hua@hisilicon.com> wrote:
> >
> >
> >
> > > -----Original Message-----
> > > From: Song Bao Hua (Barry Song)
> > > Sent: Tuesday, December 22, 2020 11:38 AM
> > > To: 'Vitaly Wool' <vitaly.wool@konsulko.com>
> > > Cc: Shakeel Butt <shakeelb@google.com>; Minchan Kim <minchan@kernel.org>;
> Mike
> > > Galbraith <efault@gmx.de>; LKML <linux-kernel@vger.kernel.org>; linux-mm
> > > <linux-mm@kvack.org>; Sebastian Andrzej Siewior <bigeasy@linutronix.de>;
> > > NitinGupta <ngupta@vflare.org>; Sergey Senozhatsky
> > > <sergey.senozhatsky.work@gmail.com>; Andrew Morton
> > > <akpm@linux-foundation.org>
> > > Subject: RE: [PATCH] zsmalloc: do not use bit_spin_lock
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: Vitaly Wool [mailto:vitaly.wool@konsulko.com]
> > > > Sent: Tuesday, December 22, 2020 11:12 AM
> > > > To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
> > > > Cc: Shakeel Butt <shakeelb@google.com>; Minchan Kim <minchan@kernel.org>;
> > > Mike
> > > > Galbraith <efault@gmx.de>; LKML <linux-kernel@vger.kernel.org>; linux-mm
> > > > <linux-mm@kvack.org>; Sebastian Andrzej Siewior <bigeasy@linutronix.de>;
> > > > NitinGupta <ngupta@vflare.org>; Sergey Senozhatsky
> > > > <sergey.senozhatsky.work@gmail.com>; Andrew Morton
> > > > <akpm@linux-foundation.org>
> > > > Subject: Re: [PATCH] zsmalloc: do not use bit_spin_lock
> > > >
> > > > On Mon, Dec 21, 2020 at 10:30 PM Song Bao Hua (Barry Song)
> > > > <song.bao.hua@hisilicon.com> wrote:
> > > > >
> > > > >
> > > > >
> > > > > > -----Original Message-----
> > > > > > From: Shakeel Butt [mailto:shakeelb@google.com]
> > > > > > Sent: Tuesday, December 22, 2020 10:03 AM
> > > > > > To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
> > > > > > Cc: Vitaly Wool <vitaly.wool@konsulko.com>; Minchan Kim
> > > > <minchan@kernel.org>;
> > > > > > Mike Galbraith <efault@gmx.de>; LKML <linux-kernel@vger.kernel.org>;
> > > > linux-mm
> > > > > > <linux-mm@kvack.org>; Sebastian Andrzej Siewior
> <bigeasy@linutronix.de>;
> > > > > > NitinGupta <ngupta@vflare.org>; Sergey Senozhatsky
> > > > > > <sergey.senozhatsky.work@gmail.com>; Andrew Morton
> > > > > > <akpm@linux-foundation.org>
> > > > > > Subject: Re: [PATCH] zsmalloc: do not use bit_spin_lock
> > > > > >
> > > > > > On Mon, Dec 21, 2020 at 12:06 PM Song Bao Hua (Barry Song)
> > > > > > <song.bao.hua@hisilicon.com> wrote:
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > > -----Original Message-----
> > > > > > > > From: Shakeel Butt [mailto:shakeelb@google.com]
> > > > > > > > Sent: Tuesday, December 22, 2020 8:50 AM
> > > > > > > > To: Vitaly Wool <vitaly.wool@konsulko.com>
> > > > > > > > Cc: Minchan Kim <minchan@kernel.org>; Mike Galbraith
> <efault@gmx.de>;
> > > > LKML
> > > > > > > > <linux-kernel@vger.kernel.org>; linux-mm <linux-mm@kvack.org>;
> Song
> > > > Bao
> > > > > > Hua
> > > > > > > > (Barry Song) <song.bao.hua@hisilicon.com>; Sebastian Andrzej
> Siewior
> > > > > > > > <bigeasy@linutronix.de>; NitinGupta <ngupta@vflare.org>; Sergey
> > > > > > Senozhatsky
> > > > > > > > <sergey.senozhatsky.work@gmail.com>; Andrew Morton
> > > > > > > > <akpm@linux-foundation.org>
> > > > > > > > Subject: Re: [PATCH] zsmalloc: do not use bit_spin_lock
> > > > > > > >
> > > > > > > > On Mon, Dec 21, 2020 at 11:20 AM Vitaly Wool
> <vitaly.wool@konsulko.com>
> > > > > > wrote:
> > > > > > > > >
> > > > > > > > > On Mon, Dec 21, 2020 at 6:24 PM Minchan Kim <minchan@kernel.org>
> > > wrote:
> > > > > > > > > >
> > > > > > > > > > On Sun, Dec 20, 2020 at 02:22:28AM +0200, Vitaly Wool wrote:
> > > > > > > > > > > zsmalloc takes bit spinlock in its _map() callback and releases
> > > > it
> > > > > > > > > > > only in unmap() which is unsafe and leads to zswap complaining
> > > > > > > > > > > about scheduling in atomic context.
> > > > > > > > > > >
> > > > > > > > > > > To fix that and to improve RT properties of zsmalloc, remove
> > > that
> > > > > > > > > > > bit spinlock completely and use a bit flag instead.
> > > > > > > > > >
> > > > > > > > > > I don't want to use such open code for the lock.
> > > > > > > > > >
> > > > > > > > > > I see from Mike's patch, recent zswap change introduced the
> lockdep
> > > > > > > > > > splat bug and you want to improve zsmalloc to fix the zswap
> bug
> > > > and
> > > > > > > > > > introduce this patch with allowing preemption enabling.
> > > > > > > > >
> > > > > > > > > This understanding is upside down. The code in zswap you are
> referring
> > > > > > > > > to is not buggy.  You may claim that it is suboptimal but there
> is
> > > > > > > > > nothing wrong in taking a mutex.
> > > > > > > > >
> > > > > > > >
> > > > > > > > Is this suboptimal for all or just the hardware accelerators?
> Sorry,
> > > > I
> > > > > > > > am not very familiar with the crypto API. If I select lzo or lz4
> as
> > > > a
> > > > > > > > zswap compressor will the [de]compression be async or sync?
> > > > > > >
> > > > > > > Right now, in crypto subsystem, new drivers are required to write
> based
> > > > on
> > > > > > > async APIs. The old sync API can't work in new accelerator drivers
> as
> > > > they
> > > > > > > are not supported at all.
> > > > > > >
> > > > > > > Old drivers are used to sync, but they've got async wrappers to
> support
> > > > async
> > > > > > > APIs. Eg.
> > > > > > > crypto: acomp - add support for lz4 via scomp
> > > > > > >
> > > > > >
> > > >
> > >
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/
> > > > > > crypto/lz4.c?id=8cd9330e0a615c931037d4def98b5ce0d540f08d
> > > > > > >
> > > > > > > crypto: acomp - add support for lzo via scomp
> > > > > > >
> > > > > >
> > > >
> > >
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/
> > > > > > crypto/lzo.c?id=ac9d2c4b39e022d2c61486bfc33b730cfd02898e
> > > > > > >
> > > > > > > so they are supporting async APIs but they are still working in
> sync
> > > mode
> > > > > > as
> > > > > > > those old drivers don't sleep.
> > > > > > >
> > > > > >
> > > > > > Good to know that those are sync because I want them to be sync.
> > > > > > Please note that zswap is a cache in front of a real swap and the
> load
> > > > > > operation is latency sensitive as it comes in the page fault path
> and
> > > > > > directly impacts the applications. I doubt decompressing synchronously
> > > > > > a 4k page on a cpu will be costlier than asynchronously decompressing
> > > > > > the same page from hardware accelerators.
> > > > >
> > > > > If you read the old paper:
> > > > >
> > > >
> > >
> https://www.ibm.com/support/pages/new-linux-zswap-compression-functionalit
> > > > y
> > > > > Because the hardware accelerator speeds up compression, looking at the
> zswap
> > > > > metrics we observed that there were more store and load requests in
> a given
> > > > > amount of time, which filled up the zswap pool faster than a software
> > > > > compression run. Because of this behavior, we set the max_pool_percent
> > > > > parameter to 30 for the hardware compression runs - this means that
> zswap
> > > > > can use up to 30% of the 10GB of total memory.
> > > > >
> > > > > So using hardware accelerators, we get a chance to speed up compression
> > > > > while decreasing cpu utilization.
> > > > >
> > > > > BTW, If it is not easy to change zsmalloc, one quick workaround we might
> > > do
> > > > > in zswap is adding the below after applying Mike's original patch:
> > > > >
> > > > > if(in_atomic()) /* for zsmalloc */
> > > > >         while(!try_wait_for_completion(&req->done);
> > > > > else /* for zbud, z3fold */
> > > > >         crypto_wait_req(....);
> > > >
> > > > I don't think I'm going to ack this, sorry.
> > > >
> > >
> > > Fair enough. And I am also thinking if we can move zpool_unmap_handle()
> > > quite after zpool_map_handle() as below:
> > >
> > >       dlen = PAGE_SIZE;
> > >       src = zpool_map_handle(entry->pool->zpool, entry->handle,
> ZPOOL_MM_RO);
> > >       if (zpool_evictable(entry->pool->zpool))
> > >               src += sizeof(struct zswap_header);
> > > +     zpool_unmap_handle(entry->pool->zpool, entry->handle);
> > >
> > >       acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
> > >       mutex_lock(acomp_ctx->mutex);
> > >       sg_init_one(&input, src, entry->length);
> > >       sg_init_table(&output, 1);
> > >       sg_set_page(&output, page, PAGE_SIZE, 0);
> > >       acomp_request_set_params(acomp_ctx->req, &input, &output,
> entry->length,
> > > dlen);
> > >       ret = crypto_wait_req(crypto_acomp_decompress(acomp_ctx->req),
> > > &acomp_ctx->wait);
> > >       mutex_unlock(acomp_ctx->mutex);
> > >
> > > -     zpool_unmap_handle(entry->pool->zpool, entry->handle);
> > >
> > > Since src is always low memory and we only need its virtual address
> > > to get the page of src in sg_init_one(). We don't actually read it
> > > by CPU anywhere.
> >
> > The below code might be better:
> >
> >         dlen = PAGE_SIZE;
> >         src = zpool_map_handle(entry->pool->zpool, entry->handle,
> ZPOOL_MM_RO);
> >         if (zpool_evictable(entry->pool->zpool))
> >                 src += sizeof(struct zswap_header);
> >
> >         acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
> >
> > +       zpool_unmap_handle(entry->pool->zpool, entry->handle);
> >
> >         mutex_lock(acomp_ctx->mutex);
> >         sg_init_one(&input, src, entry->length);
> >         sg_init_table(&output, 1);
> >         sg_set_page(&output, page, PAGE_SIZE, 0);
> >         acomp_request_set_params(acomp_ctx->req, &input, &output,
> entry->length, dlen);
> >         ret = crypto_wait_req(crypto_acomp_decompress(acomp_ctx->req),
> &acomp_ctx->wait);
> >         mutex_unlock(acomp_ctx->mutex);
> >
> > -       zpool_unmap_handle(entry->pool->zpool, entry->handle);
> 
> I don't see how this is going to work since we can't guarantee src
> will be a valid pointer after the zpool_unmap_handle() call, can we?
> Could you please elaborate?

A valid pointer is for cpu to read and write. Here, cpu doesn't read
and write it, we only need to get page struct from the address.

void sg_init_one(struct scatterlist *sg, const void *buf, unsigned int buflen)
{
	sg_init_table(sg, 1);
	sg_set_buf(sg, buf, buflen);
}

static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
			      unsigned int buflen)
{
#ifdef CONFIG_DEBUG_SG
	BUG_ON(!virt_addr_valid(buf));
#endif
	sg_set_page(sg, virt_to_page(buf), buflen, offset_in_page(buf));
}

sg_init_one() is always using an address which has a linear mapping
with physical address.
So once we get the value of src, we can get the page struct.

src has a linear mapping with physical address. It doesn't require
page table walk which vmalloc_to_page() wants.

The req only requires page to initialize sg table, I think if
we are going to use a cpu-based (de)compression, the crypto
driver will kmap it again.

> 
> ~Vitaly

Thanks
Barry