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. 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. Eric
