Hi Will, What's your opinion about my explanation? Thanks! On 2014/5/20 11:22, Wang Nan wrote: > On 2014/5/20 0:09, Will Deacon wrote: >> On Mon, May 19, 2014 at 02:54:03AM +0100, Wang Nan wrote: >>> When SPARSEMEM and CRASH_DUMP both selected, simple pfn_valid prevents >>> the second kernel ioremap first kernel's memory if the address falls >>> into second kernel section. This limitation requires the second kernel >>> occupies a full section, and elfcorehdr must resides in another section. >>> >>> This patch makes crash dump kernel use strict pfn_valid, removes such >>> limitation. >>> >>> For example: >>> >>> For a platform with SECTION_SIZE_BITS == 28 (256MiB) and >>> crashkernel=128M at 0x28000000 in kernel cmdline, the second >>> kernel is loaded at 0x28000000. Kexec puts elfcorehdr at >>> 0x2ff00000, and passes 'elfcorehdr=0x2ff00000 mem=130048K' to >>> second kernel. When second kernel start, it tries to use >>> ioremap to retrive its elfcorehrd. In this case, elfcodehdr is at the >>> same section of the second kernel, pfn_valid will recongnize >>> the page as valid, so ioremap will refuse to map it. >> >> So isn't the issue here that you're passing an incorrect mem= parameter >> to the crash kernel? >> > > mem= parameter is generated by kexec-tools according to /proc/iomem, it is the size > of reserved memory minus 1MiB. So I think what you mean is I passing an incorrect > crashkernel= parameter? > > I'll explain limitations on crash kernel reserved memory in the case of SPARSEMEM > enabled, and show how *impractical* the 'correct' crashkernel will be. > > Use realview board for example. > > Limitation 1: crash kernel reservation kernel must be aligned with 0x08000000 (128MiB). > > This is because zImage determine final kernel address by (pc & 0xf8000000). If, > for example, set crashkernel=64M at 0x29000000, then the second kernel itself > overwrites first kernel's memory. We'll lost some memory in /proc/vmcore. > > Limitation 2: crash kernel must resides in different section with the first kernel. > > This is because the second kernel use ioremap for accessing first kernel's memory, > and arm prevent a valid pfn be ioremapped. Which means a whole section must be reserved > for the secton kernel. On realview, which is 256MiB. > > Limitation 3: the last 1MiB of reserved memory must be ioremappable. > > This is because the second kernel depeneds kexec-tools passing an elfheader as > 'elfcorehdr' to instructs it generating /proc/vmcore. See fs/proc/vmcore.c. Kexec-tools > simply uses the last 1MiB for it. The second kernel use ioremap to access it, force > the header be put in another section. > > In realview board, the only possible correct setting should be 'crashkernel=257M at 0x20000000'. > However, realview has only 1GiB memory, crash kernel consumes a quarter plus 1MiB. In addition, even > set this parameter, crash kernel is still unusable because: > > crashkernel reservation failed - memory is in use (0x20000000) > >> Will >> > > > > _______________________________________________ > kexec mailing list > kexec at lists.infradead.org > http://lists.infradead.org/mailman/listinfo/kexec >
