Hi Catalin, On 2020/7/29 23:20, Catalin Marinas wrote: > On Wed, Jul 29, 2020 at 10:14:32PM +0800, chenzhou wrote: >> On 2020/7/29 19:58, Catalin Marinas wrote: >>> On Wed, Jul 29, 2020 at 11:52:39AM +0800, chenzhou wrote: >>>> How about like this: >>>> 1. For ZONE_DMA issue, use Bhupesh's solution, keep the crashkernel= >>>> behaviour to ZONE_DMA allocations. >>>> 2. For this patch series, make the reserve_crashkernel_low() to >>>> ZONE_DMA allocations. >>> So you mean rebasing your series on top of Bhupesh's? I guess you can >>> combine the two, I really don't care which way as long as we fix both >>> issues and agree on the crashkernel= semantics. I think with some tweaks >>> we can go with your series alone. >>> >>> IIUC from the x86 code (especially the part you #ifdef'ed out for >>> arm64), if ",low" is not passed (so just standard crashkernel=X), it >>> still allocates sufficient low memory for the swiotlb in ZONE_DMA. The >>> rest can go in a high region. Why can't we do something similar on >>> arm64? Of course, you can keep the ",low" argument for explicit >>> allocation but I don't want to mandate it. >> It is a good idea to combine the two. >> >> For parameter crashkernel=X, we do like this: >> 1. allocate some low memory in ZONE_DMA(or ZONE_DMA32 if CONFIG_ZONE_DMA=n) >> 2. allocate X size memory in a high region >> >> ",low" argument can be used to specify the low memory. >> >> Do i understand correctly? > Yes, although we could follow the x86 approach: > > 1. Try low (ZONE_DMA for arm64) allocation, fallback to high allocation > if it fails. > > 2. If crash_base is outside ZONE_DMA, call reserve_crashkernel_low() > which either honours the ,low option or allocates some small amount > in ZONE_DMA. > > If at some point we have platforms failing step 2, we'll look at > changing ZONE_DMA to the full 4GB on non-RPi4 platforms. > > It looks to me like x86 ignores the ,low option if the first step > managed to get some low memory. Shall we do the same on arm64? Yes, we could do like this. > >>> So with an implicit ZONE_DMA allocation similar to the x86 one, we >>> probably don't need Bhupesh's series at all. In addition, we can limit >>> crashkernel= to the first 4G with a fall-back to high like x86 (not sure >>> if memblock_find_in_range() is guaranteed to search in ascending order). >>> I don't think we need an explicit ",high" annotation. >>> >>> So with the above, just a crashkernel=1G gives you at least 256MB in >>> ZONE_DMA followed by the rest anywhere, with a preference for >>> ZONE_DMA32. This way we can also keep the reserve_crashkernel_low() >>> mostly intact from x86 (less #ifdef's). >> Yes. We can let crashkernel=X try to reserve low memory and fall back to use high memory >> if failing to find a low range. > The only question is whether we need to preserve some more ZONE_DMA on > the current system. If for example we pass a crashkernel=512M and some > cma=, we may end up with very little free memory in ZONE_DMA. That's > mostly an issue for RPi4 since other platforms would work with > ZONE_DMA32. We could add a threshold and go for high allocation directly > if the required size is too large. Ok. I will think about the threshold in the next version and make the value be 1/2 or 1/3 of the ZONE_DMA. > >> About the function reserve_crashkernel_low(), if we put it in arch/arm64, there is some common >> code with x86_64. Some suggestions about this? > If we can use this function almost intact, just move it in a common > place. But if it gets sprinkled with #ifdef CONFIG_ARM64, I'd rather > duplicate it. I'd still prefer to move it to a common place if possible. > > You can go a step further and also move the x86 reserve_crashkernel() to > common code. I don't think there a significant difference between arm64 > and x86 here. You'd have to define arch-specific specific > CRASH_ADDR_LOW_MAX etc. I will take these into account and send the next version recently. > > Also patches moving code should not have any functional change. The > CRASH_ALIGN change from 16M to 2M on x86 should be a separate patch as > it needs to be acked by the x86 maintainers (IIRC, Ingo only acked the > function move if there was no functional change; CRASH_ALIGN is used for > the start address, not just alignment, on x86). > Thanks, Chen Zhou
