On 05/05/22 at 03:20pm, Catalin Marinas wrote: > On Thu, May 05, 2022 at 11:00:19AM +0800, Baoquan He wrote: > > On 05/03/22 at 11:00pm, Catalin Marinas wrote: > > > So, to recap, IIUC you are fine with: > > > > > > crashkernel=Y - allocate within ZONE_DMA with fallback > > > above with a default in ZONE_DMA (like > > > x86, 256M or swiotlb size) > > > > Ack to this one. > > > > > > > crashkernel=Y,high - allocate from above ZONE_DMA > > > > Not exactly. If there's only ZONE_DMA, crashkernel,high will > > be reserved in ZONE_DMA, and crashkernel,low will be ignored. > > Other than this, ack. > > Yes, that's fine. > > > > crashkernel=Y,low - allocate within ZONE_DMA > > > > Ack to this one. > > > > > > 'crashkernel' overrides the high and low while the latter two can be > > > passed independently. > > > > crashkernel=,high can be passed independently, then a crashkernel=,low > > is needed implicitly. If people don't want crashkernel=,low > > explicitly, crashkernel=0,low need be specified. > > I find this complicating the interface. I don't know the background to > the x86 implementation but we diverge already on arm64 since we talk > about ZONE_DMA rather than 4G limit (though for most platforms these > would be the same). > > I guess we could restate the difference between crashkernel= and > crashkernel=,high as the hint to go for allocation above ZONE_DMA first. Yes, rethinking about this, we can make a straightforward and simpler crashkernel=,high|,low on arm64, namely asking for user to clearly specify them. During maintenance of crashkernel= parameter in our distros, we found crashkernel=xM is used mostly since most of systems can be satisfied with 256M or a little more for kdump. While on some big end servers, 1G or more crashkernel memory is needed. In this case, crashkernel=,high is taken. We don't want to reserve so much low memory during system running while just waiting in case rare crash happened. crashkernel=,high is rarely used, so making it simple and not so flexible is not so bad. We can improve it later with justification. > > > An independent crashkernel=,low makes no sense. Crashkernel=,low > > should be paird with crashkernel=,high. > > You could argue that crashkernel=,low gives the current crashkernel= > behaviour, i.e. either all within ZONE_DMA or fail to allocate. So it > may have some value on its own. Yes, crashkernel=,low has the same behaviour as the current crashkernel= if we decide not to add fallback mechanism to it. The purpose of crahskernel=,low is to assist crashkernel=,high to get kdump kernel boot up with satisfing DMA allocation. While allowing independent crashkernel=,low will add it another mission, limiting crashkernel only reserved in low memory. Up to now, we don't see the need for that. > > > My personal opinion according to the existed senmantics on x86. > > Otherwise, the guidance of crashkernel= |,high|,low reservation > > will be complicated to write. > > It's more that I find the current semantics unnecessarily confusing. But > even reading the x86_64 text it's not that clear. For example the > default low allocation for crashkernel= and crashkernel=,high is only > mentioned in the crashkernel=,low description. Yeah, we can improve those document if insufficiency is found. By the way, with my observation, crashkernel= with fallback meet 99% of our needs. If people really need more than 512M memory or more, then please consider crashkernel=,high. Basically on servers, low memory is limited, while high memory is very big. So I agree with you that we can make it step by step, firstly adding basic crashkernel=,high and ,low support. We can add those complicated cases later.
