Re: kexec-ed kernel possibly needing low memory

[David Vrabel <david.vrabel@citrix.com>]

On 14/11/14 10:00, Jan Beulich wrote:
> David,
> 
> we're being approached with the situation where a disk driver in the
> kexec-ed kernel needs memory below 4G in order to perform DMA
> (e.g. for the swiotlb to be set up). Linux not so long ago invented a
> two area approach, which doesn't fit with the current single
> KEXEC_RANGE_MA_CRASH area obtainable via
> KEXEC_CMD_kexec_get_range.
> 
> I see multiple options
> - do no change at all; the user can deal with this by explicitly
>   specifying an area below 4G via "crashkernel="

This is what we do.

> - add KEXEC_RANGE_MA_CRASH_LOW

If you choose this option, it would be preferable to support N (which
might be 2) arbitrary crash regions rather than specifying that one
region is always low memory.  I would suggest combining this with a way
to specify the bounds of each region.

e.g., crashkernel=32M@<4G,128M

I wouldn't go this route unless you actually need a large crash region
that would use up too much low memory otherwise.

> - when not asked for a specific address, always allocate the (single)
>   area below 4G if there is enough space

I don't think the default location should change.  A user might have
specified  a large crash region that might use up most of low memory.

> - provide a means to request allocating the (single) area below 4G
>   (or perhaps more generically below a certain boundary) without
>   requiring an exact address to be specified

This sounds ok.

> Do you have any preference here, or do you see other viable
> alternatives?

My preference would be option 1 since it already works, then option 4.

David