On Wed, Oct 07, 2020 at 12:37:49PM +0530, Bhupesh Sharma wrote: > On Tue, Oct 6, 2020 at 11:30 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > On Mon, Oct 05, 2020 at 11:12:10PM +0530, Bhupesh Sharma wrote: > > > I think my earlier email with the test results on this series bounced > > > off the mailing list server (for some weird reason), but I still see > > > several issues with this patchset. I will add specific issues in the > > > review comments for each patch again, but overall, with a crashkernel > > > size of say 786M, I see the following issue: > > > > > > # cat /proc/cmdline > > > BOOT_IMAGE=(hd7,gpt2)/vmlinuz-5.9.0-rc7+ root=<..snip..> rd.lvm.lv=<..snip..> crashkernel=786M > > > > > > I see two regions of size 786M and 256M reserved in low and high > > > regions respectively, So we reserve a total of 1042M of memory, which > > > is an incorrect behaviour: > > > > > > # dmesg | grep -i crash > > > [ 0.000000] Reserving 256MB of low memory at 2816MB for crashkernel (System low RAM: 768MB) > > > [ 0.000000] Reserving 786MB of memory at 654158MB for crashkernel (System RAM: 130816MB) > > > [ 0.000000] Kernel command line: BOOT_IMAGE=(hd2,gpt2)/vmlinuz-5.9.0-rc7+ root=/dev/mapper/rhel_ampere--hr330a--03-root ro rd.lvm.lv=rhel_ampere-hr330a-03/root rd.lvm.lv=rhel_ampere-hr330a-03/swap crashkernel=786M cma=1024M > > > > > > # cat /proc/iomem | grep -i crash > > > b0000000-bfffffff : Crash kernel (low) > > > bfcbe00000-bffcffffff : Crash kernel > > > > As Chen said, that's the intended behaviour and how x86 works. The > > requested 768M goes in the high range if there's not enough low memory > > and an additional buffer for swiotlb is allocated, hence the low 256M. > > I understand, but why 256M (as low) for arm64? x86_64 setups usually > have more system memory available as compared to several commercially > available arm64 setups. So is the intent, just to keep the behavior > similar between arm64 and x86_64? Similar in the sense of the fallback to high memory and some low memory allocation but the amounts can vary per architecture. > Should we have a CONFIG option / bootarg to help one select the max > 'low_size'? Currently the ' low_size' value is calculated as: > > /* > * two parts from kernel/dma/swiotlb.c: > * -swiotlb size: user-specified with swiotlb= or default. > * > * -swiotlb overflow buffer: now hardcoded to 32k. We round it > * to 8M for other buffers that may need to stay low too. Also > * make sure we allocate enough extra low memory so that we > * don't run out of DMA buffers for 32-bit devices. > */ > low_size = max(swiotlb_size_or_default() + (8UL << 20), 256UL << 20); > > Since many arm64 boards ship with swiotlb=0 (turned off) via kernel > bootargs, the low_size, still ends up being 256M in such cases, > whereas this 256M can be used for some other purposes - so should we > be limiting this to 64M and failing the crash kernel allocation > request (gracefully) otherwise? I think it makes sense to set a low_size = 0 if swiotlb_size_or_default() is 0. The assumption would be that if the main kernel doesn't need an swiotlb, the crashdump one wouldn't need it either. But this probably needs the ZONE_DMA for non-RPi4 platforms addressed as well (expanded to the whole ZONE_DMA32). -- Catalin
