From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Date: Wed, 7 Oct 2020 17:33:19 +0100 From: Catalin Marinas Subject: Re: [PATCH v12 0/9] support reserving crashkernel above 4G on arm64 kdump Message-ID: <20201007163319.GS3462@gaia> References: <20200907134745.25732-1-chenzhou10@huawei.com> <20201005170937.GA14576@gaia> <20201006180012.GB31946@C02TF0J2HF1T.local> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "kexec" Errors-To: kexec-bounces+dwmw2=infradead.org@lists.infradead.org To: Bhupesh Sharma Cc: wangkefeng.wang@huawei.com, Linux Doc Mailing List , Chen Zhou , huawei.libin@huawei.com, guohanjun@huawei.com, Will Deacon , Baoquan He , Jonathan Corbet , Ingo Molnar , RuiRui Yang , John Donnelly , Arnd Bergmann , xiexiuqi@huawei.com, Simon Horman , Thomas Gleixner , linux-arm-kernel , kexec mailing list , Linux Kernel Mailing List , Rob Herring , James Morse , Prabhakar Kushwaha , nsaenzjulienne@suse.de On Wed, Oct 07, 2020 at 12:37:49PM +0530, Bhupesh Sharma wrote: > On Tue, Oct 6, 2020 at 11:30 PM Catalin Marinas 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 _______________________________________________ kexec mailing list kexec@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kexec