linux-kernel - RE: [PATCH v3 5/5] mm/slub: do not create dma-kmalloc if no managed pages in DMA zone

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Message-ID: <67ff367b7a90452f8c009707a00e67b2@AcuMS.aculab.com>
Date:   Wed, 15 Dec 2021 11:51:50 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Vlastimil Babka' <vbabka@...e.cz>, Christoph Hellwig <hch@....de>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>
CC:     Baoquan He <bhe@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "cl@...ux.com" <cl@...ux.com>,
        "John.p.donnelly@...cle.com" <John.p.donnelly@...cle.com>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: RE: [PATCH v3 5/5] mm/slub: do not create dma-kmalloc if no managed
 pages in DMA zone

From: Vlastimil Babka
> Sent: 15 December 2021 10:34
> 
> On 12/15/21 08:27, Christoph Hellwig wrote:
> > On Wed, Dec 15, 2021 at 07:03:35AM +0000, Hyeonggon Yoo wrote:
> >> I'm not sure that allocating from ZONE_DMA32 instead of ZONE_DMA
> >> for kdump kernel is nice way to solve this problem.
> >
> > What is the problem with zones in kdump kernels?
> 
> My understanding is that kdump kernel can only use physical memory that it
> got reserved by the main kernel, and the main kernel will reserve some block
> of memory that doesn't include any pages from ZONE_DMA (first 16MB of
> physical memory or whatnot). 
...

Is there still any support for any of the very old hardware that could only
support 24bit DMA?

I think the AMD PCnet-ISA and PCnet-PCI ethernet (lance) were both 32bit masters.
(I don't remember ever having to worry about physical addresses.)
I'm sure I remember some old SCSI boards only being able to do 24bit DMA.
But I can't remember which bus interface they were.
Unlikely to be ISA because it has always been hard to get a motherboard
DMA channel into 'cascade mode'.

Might have been some EISA boards - anyone still use those?
So we are left with early PCI boards.

It really is worth looking at what actually needs it at all.

	David

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