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