Hello Eric, ? 2013?05?25? 06:44, Eric W. Biederman ??: > Vivek Goyal <vgoyal at redhat.com> writes: > >> On Fri, May 24, 2013 at 05:06:26PM +0200, Michael Holzheu wrote: >>> Hello Vivek, >>> >>> On Fri, 24 May 2013 10:36:44 -0400 >>> Vivek Goyal <vgoyal at redhat.com> wrote: >>> >>> [snip] >>> >>>> Sorry, I don't understand the problem. If we swapped low memory and >>>> crash reserved memory, that should have been taken care by prepared >>>> ELF headers so that we map the right pfns. In x86 we swap 640K of low >>>> memory with 640K of memory in reserved and we take care of this by >>>> preparing elf headers accordingly. >>>> >>>> So why s390 can't do the same thing? >>> >>> I am not sure if I understand this. Currently we create the ELF >>> header in a way that we have virtual=real. In the copy_oldmem_page() we >>> do the swap so that for the /proc/vmcore code it looks like contiguous >>> non-swapped memory. >>> >>> One reason why I thought this was necessary was that /dev/oldmem >>> also uses the function and it should provide linear memory access like >>> it is on the live system with /dev/mem. >>> >>> Is that implementation incorrect? >> >> [ CC Andrew. Keep him in loop for all kernel kdump patches as all kdump >> patches are routed through him ]. >> >> [ CC Eric Biederman ] >> >> Looking at the code, looks like /dev/oldmem is broken. It does not know >> anything about swap of any of the memory areas and it will simply >> return the contents of page frame asked. And this has been like this >> since the beginning. >> >> I have always questioned the utility of /dev/oldmem. Atleast I am not >> aware of any tool making use of it. >> >> If we want to fix it, then somebow all the swapped memory region info >> needs to be communicated to second kernel so that read_oldmem() can >> do the mapping correctly and we really don't have any mechanism for >> that. (I am assuming that in s390 you must have hardcoded the regions >> of memory which are always swapped). >> >> As /proc/vmcore is the most used and useful interface, I prefer that >> we swap memory and put that info in elf headers. For /dev/oldme, I >> don't mind if we leave it as it is. If somebody really cares, then >> I guess we need to write a new command line option which /dev/mem >> can parse and which tells it about swaps so that /dev/oldmem can >> map things correctly. (This is better than hardcoding things). >> >> Eric, do you have any thoughts on this. > > I don't think anyone actually uses /dev/oldmem. I would like to cite > the s390 confusion as proof but I don't think that quite works. > > I think the solution is for someone to send a patch removing /dev/oldmem > as an unused piece of code. That will also move us in the direction of > resolving HPAs concerns. I think I can try this. > > The function copy_oldmem_page also concerns me. I don't have a clue why > we duplicate that function on every architecutre in a slightly different > form. There should be enough abstractions in the kernel to make that > unnecessary. I would be glad to see that function go, and remove the > possibility of confusion that happened on s390. You mean we should have a common copy_oldmem_page for all architectures? And just like vivek said above, for s390, we should put the swap info in the elf headers instead of doing that in copy_oldmem_page. Zhang
