linux-kernel - Re: [LSF/MM TOPIC NOTES] x86 ZONE

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Date:   Fri, 27 Apr 2018 16:14:56 +0000
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>, linux-mm@...ck.org,
        mhocko@...nel.org, cl@...ux.com, Jan Kara <jack@...e.cz>,
        matthew@....cx, x86@...nel.org, luto@...capital.net,
        martin.petersen@...cle.com, jthumshirn@...e.de, broonie@...nel.org,
        Juergen Gross <jgross@...e.com>, linux-spi@...r.kernel.org,
        Joerg Roedel <joro@...tes.org>, linux-scsi@...r.kernel.org,
        Dan Carpenter <dan.carpenter@...cle.com>,
        linux-kernel@...r.kernel.org,
        "lsf-pc@...ts.linux-foundation.org" 
        <lsf-pc@...ts.linux-foundation.org>
Subject: Re: [LSF/MM TOPIC NOTES] x86 ZONE_DMA love

On Thu, Apr 26, 2018 at 10:35:56PM -0700, Christoph Hellwig wrote:
> On Thu, Apr 26, 2018 at 09:54:06PM +0000, Luis R. Rodriguez wrote:
> > In practice if you don't have a floppy device on x86, you don't need ZONE_DMA,
> 
> I call BS on that, 

I did not explain though that it was not me who claimed this though.
The list displayed below is the result of trying to confirm/deny this,
and what could be done, and also evaluating if there is *any* gain
about doing something about it.

But curious, on a standard qemu x86_x64 KVM guest, which of the
drivers do we know for certain *are* being used from the ones
listed?

What about Xen guests, I wonder?

> and you actually explain later why it it BS due
> to some drivers using it more explicitly.

Or implicitly. The list I showed is the work to show that the users
of GFP_DMA on x86 is *much* more wide spread than expected from the
above claim.

I however did not also answer the above qemu x86_64 question, but
would be good to know. Note I stated that the claim was *in practice*.

> But even more importantly
> we have plenty driver using it through dma_alloc_* and a small DMA
> mask, and they are in use 

Do we have a list of users for x86 with a small DMA mask?
Or, given that I'm not aware of a tool to be able to look
for this in an easy way, would it be good to find out which
x86 drivers do have a small mask?

> - we actually had a 4.16 regression due to them.

Ah what commit was the culprit? Is that fixed already? If so what
commit?

> > SCSI is *severely* affected:
> 
> Not really.  We have unchecked_isa_dma to support about 4 drivers,

Ah very neat:

  * CONFIG_CHR_DEV_OSST - "SCSI OnStream SC-x0 tape support"
  * CONFIG_SCSI_ADVANSYS - "AdvanSys SCSI support"
  * CONFIG_SCSI_AHA1542 - "Adaptec AHA1542 support"
  * CONFIG_SCSI_ESAS2R - "ATTO Technology's ExpressSAS RAID adapter driver"

> and less than a hand ful of drivers doing stupid things, which can
> be fixed easily, and just need a volunteer.

Care to list what needs to be done? Can an eager beaver student do it?

> > That's the end of the review of all current explicit callers on x86.
> > 
> > # dma_alloc_coherent_gfp_flags() and dma_generic_alloc_coherent()
> > 
> > dma_alloc_coherent_gfp_flags() and dma_generic_alloc_coherent() set
> > GFP_DMA if if (dma_mask <= DMA_BIT_MASK(24))
> 
> All that code is long gone and replaced with dma-direct.  Which still
> uses GFP_DMA based on the dma mask, though - see above.

And that's mostly IOMMU code, on the alloc() dma_map_ops.

  Luis