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:

quoted

On Wed, Dec 15, 2021 at 07:03:35AM +0000, Hyeonggon Yoo wrote:

quoted

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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