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From: Catalin Marinas <catalin.marinas@arm.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Arnd Bergmann <arnd@arndb.de>, Will Deacon <will@kernel.org>,
	Marc Zyngier <maz@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Ard Biesheuvel <ardb@kernel.org>,
	Isaac Manjarres <isaacmanjarres@google.com>,
	Saravana Kannan <saravanak@google.com>,
	linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 2/2] treewide: Add the __GFP_PACKED flag to several non-DMA kmalloc() allocations
Date: Tue, 1 Nov 2022 17:32:14 +0000	[thread overview]
Message-ID: <Y2FYHiBjQK/v2+kV@arm.com> (raw)
In-Reply-To: <20221101172416.GB20381@lst.de>

On Tue, Nov 01, 2022 at 06:24:16PM +0100, Christoph Hellwig wrote:
> On Tue, Nov 01, 2022 at 05:19:46PM +0000, Catalin Marinas wrote:
> > 
> > The main downside of bouncing is mobile phones where those vendors are
> > in the habit of passing noswiotlb on the kernel command line or they
> > want a very small bounce buffer (hard to pick a universally small size).
> > I guess we can go ahead with this and, depending on how bad the problem
> > is, we can look at optimising swiotlb to use a kmem_cache (or aligned
> > kmalloc) as fall-back for bouncing.
> 
> Theses phones setups are completely broken already.  There is a reason
> why it needs a debug option to disable swiotlb, and that reason is
> that Linux guarantees that the 32-bit DMA always works.  If they force
> swiotlb off they can keep the pieces as this is not a supported
> configuration.

There's also the case of low-end phones with all RAM below 4GB and arm64
doesn't allocate the swiotlb. Not sure those vendors would go with a
recent kernel anyway.

So the need for swiotlb now changes from 32-bit DMA to any DMA
(non-coherent but we can't tell upfront when booting, devices may be
initialised pretty late).

> > As a less than optimal solution, we can force bouncing for the whole
> > list if any of the sg elements is below the alignment size. Hopefully we
> > won't have many such mixed size cases.
> 
> I suspect we won't see any such cases.  The scatterlist is usually used
> for large data transfers, and many devices won't like unaligned buffers
> for SG operations to start with.  So I think it is a perfectly fine
> tradeoff.

I'll give it a try this week, hopefully post patches soon(ish).

-- 
Catalin

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WARNING: multiple messages have this Message-ID (diff)
From: Catalin Marinas <catalin.marinas@arm.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Arnd Bergmann <arnd@arndb.de>, Will Deacon <will@kernel.org>,
	Marc Zyngier <maz@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Ard Biesheuvel <ardb@kernel.org>,
	Isaac Manjarres <isaacmanjarres@google.com>,
	Saravana Kannan <saravanak@google.com>,
	linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 2/2] treewide: Add the __GFP_PACKED flag to several non-DMA kmalloc() allocations
Date: Tue, 1 Nov 2022 17:32:14 +0000	[thread overview]
Message-ID: <Y2FYHiBjQK/v2+kV@arm.com> (raw)
In-Reply-To: <20221101172416.GB20381@lst.de>

On Tue, Nov 01, 2022 at 06:24:16PM +0100, Christoph Hellwig wrote:
> On Tue, Nov 01, 2022 at 05:19:46PM +0000, Catalin Marinas wrote:
> > 
> > The main downside of bouncing is mobile phones where those vendors are
> > in the habit of passing noswiotlb on the kernel command line or they
> > want a very small bounce buffer (hard to pick a universally small size).
> > I guess we can go ahead with this and, depending on how bad the problem
> > is, we can look at optimising swiotlb to use a kmem_cache (or aligned
> > kmalloc) as fall-back for bouncing.
> 
> Theses phones setups are completely broken already.  There is a reason
> why it needs a debug option to disable swiotlb, and that reason is
> that Linux guarantees that the 32-bit DMA always works.  If they force
> swiotlb off they can keep the pieces as this is not a supported
> configuration.

There's also the case of low-end phones with all RAM below 4GB and arm64
doesn't allocate the swiotlb. Not sure those vendors would go with a
recent kernel anyway.

So the need for swiotlb now changes from 32-bit DMA to any DMA
(non-coherent but we can't tell upfront when booting, devices may be
initialised pretty late).

> > As a less than optimal solution, we can force bouncing for the whole
> > list if any of the sg elements is below the alignment size. Hopefully we
> > won't have many such mixed size cases.
> 
> I suspect we won't see any such cases.  The scatterlist is usually used
> for large data transfers, and many devices won't like unaligned buffers
> for SG operations to start with.  So I think it is a perfectly fine
> tradeoff.

I'll give it a try this week, hopefully post patches soon(ish).

-- 
Catalin


  reply	other threads:[~2022-11-01 17:33 UTC|newest]

Thread overview: 74+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-10-25 20:52 [PATCH v2 0/2] mm: Allow kmalloc() allocations below ARCH_KMALLOC_MINALIGN Catalin Marinas
2022-10-25 20:52 ` Catalin Marinas
2022-10-25 20:52 ` [PATCH v2 1/2] mm: slab: Introduce __GFP_PACKED for smaller kmalloc() alignments Catalin Marinas
2022-10-25 20:52   ` Catalin Marinas
2022-10-26  6:39   ` Greg Kroah-Hartman
2022-10-26  6:39     ` Greg Kroah-Hartman
2022-10-26  8:39     ` Catalin Marinas
2022-10-26  8:39       ` Catalin Marinas
2022-10-26  9:49       ` Greg Kroah-Hartman
2022-10-26  9:49         ` Greg Kroah-Hartman
2022-10-26  9:58         ` Catalin Marinas
2022-10-26  9:58           ` Catalin Marinas
2022-10-27 12:11   ` Hyeonggon Yoo
2022-10-27 12:11     ` Hyeonggon Yoo
2022-10-28  7:32     ` Catalin Marinas
2022-10-28  7:32       ` Catalin Marinas
2022-10-25 20:52 ` [PATCH v2 2/2] treewide: Add the __GFP_PACKED flag to several non-DMA kmalloc() allocations Catalin Marinas
2022-10-25 20:52   ` Catalin Marinas
2022-10-26  6:50   ` Greg Kroah-Hartman
2022-10-26  6:50     ` Greg Kroah-Hartman
2022-10-26  9:48     ` Catalin Marinas
2022-10-26  9:48       ` Catalin Marinas
2022-10-26 12:59       ` Greg Kroah-Hartman
2022-10-26 12:59         ` Greg Kroah-Hartman
2022-10-26 17:09         ` Catalin Marinas
2022-10-26 17:09           ` Catalin Marinas
2022-10-26 17:21           ` Greg Kroah-Hartman
2022-10-26 17:21             ` Greg Kroah-Hartman
2022-10-26 17:46       ` Linus Torvalds
2022-10-26 17:46         ` Linus Torvalds
2022-10-27 22:29         ` Catalin Marinas
2022-10-27 22:29           ` Catalin Marinas
2022-10-28  9:37           ` Greg Kroah-Hartman
2022-10-28  9:37             ` Greg Kroah-Hartman
2022-10-28  9:37             ` Greg Kroah-Hartman
2022-10-28  9:37               ` Greg Kroah-Hartman
2022-10-30  8:47               ` Christoph Hellwig
2022-10-30  8:47                 ` Christoph Hellwig
2022-10-30  9:02                 ` Greg Kroah-Hartman
2022-10-30  9:02                   ` Greg Kroah-Hartman
2022-10-30  9:13                   ` Christoph Hellwig
2022-10-30  9:13                     ` Christoph Hellwig
2022-10-30 16:43                     ` Catalin Marinas
2022-10-30 16:43                       ` Catalin Marinas
2022-11-01 10:59                       ` Christoph Hellwig
2022-11-01 10:59                         ` Christoph Hellwig
2022-11-01 17:19                         ` Catalin Marinas
2022-11-01 17:19                           ` Catalin Marinas
2022-11-01 17:24                           ` Christoph Hellwig
2022-11-01 17:24                             ` Christoph Hellwig
2022-11-01 17:32                             ` Catalin Marinas [this message]
2022-11-01 17:32                               ` Catalin Marinas
2022-11-01 17:39                               ` Christoph Hellwig
2022-11-01 17:39                                 ` Christoph Hellwig
2022-11-01 19:10                                 ` Isaac Manjarres
2022-11-01 19:10                                   ` Isaac Manjarres
2022-11-02 11:05                                   ` Catalin Marinas
2022-11-02 11:05                                     ` Catalin Marinas
2022-11-02 20:50                                     ` Isaac Manjarres
2022-11-02 20:50                                       ` Isaac Manjarres
2022-11-01 18:14                           ` Robin Murphy
2022-11-01 18:14                             ` Robin Murphy
2022-11-02 13:10                             ` Catalin Marinas
2022-11-02 13:10                               ` Catalin Marinas
2022-10-30  8:46           ` Christoph Hellwig
2022-10-30  8:46             ` Christoph Hellwig
2022-10-30  8:44         ` Christoph Hellwig
2022-10-30  8:44           ` Christoph Hellwig
2022-11-03 16:15       ` Vlastimil Babka
2022-11-03 16:15         ` Vlastimil Babka
2022-11-03 18:03         ` Catalin Marinas
2022-11-03 18:03           ` Catalin Marinas
2022-10-26  6:54 ` [PATCH v2 0/2] mm: Allow kmalloc() allocations below ARCH_KMALLOC_MINALIGN Greg Kroah-Hartman
2022-10-26  6:54   ` Greg Kroah-Hartman

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