Vivek Goyal <vgoyal at redhat.com> writes: > On Tue, Mar 19, 2013 at 03:38:45PM -0700, Eric W. Biederman wrote: >> HATAYAMA Daisuke <d.hatayama at jp.fujitsu.com> writes: >> >> > If there's some vmcore object that doesn't satisfy page-size boundary >> > requirement, remap_pfn_range() fails to remap it to user-space. >> > >> > Objects that posisbly don't satisfy the requirement are ELF note >> > segments only. The memory chunks corresponding to PT_LOAD entries are >> > guaranteed to satisfy page-size boundary requirement by the copy from >> > old memory to buffer in 2nd kernel done in later patch. >> > >> > This patch doesn't copy each note segment into the 2nd kernel since >> > they amount to so large in total if there are multiple CPUs. For >> > example, current maximum number of CPUs in x86_64 is 5120, where note >> > segments exceed 1MB with NT_PRSTATUS only. >> >> So you require the first kernel to reserve an additional 20MB, instead >> of just 1.6MB. 336 bytes versus 4096 bytes. >> >> That seems like completely the wrong tradeoff in memory consumption, >> filesize, and backwards compatibility. > > Agreed. > > So we already copy ELF headers in second kernel's memory. If we start > copying notes too, then both headers and notes will support mmap(). The only real is it could be a bit tricky to allocate all of the memory for the notes section on high cpu count systems in a single allocation. > For mmap() of memory regions which are not page aligned, we can map > extra bytes (as you suggested in one of the mails). Given the fact > that we have one ELF header for every memory range, we can always modify > the file offset where phdr data is starting to make space for mapping > of extra bytes. Agreed ELF file offset % PAGE_SIZE should == physical address % PAGE_SIZE to make mmap work. > That way whole of vmcore should be mmappable and user does not have > to worry about reading part of the file and mmaping the rest. That sounds simplest. If core counts on the high end do more than double every 2 years we might have a problem. Otherwise making everything mmapable seems easy and sound. Eric
