On 2016/7/13 13:07, Eric W. Biederman wrote: > zhong jiang <zhongjiang at huawei.com> writes: > >> On 2016/7/12 23:19, Eric W. Biederman wrote: >>> zhongjiang <zhongjiang at huawei.com> writes: >>> >>>> From: zhong jiang <zhongjiang at huawei.com> >>>> >>>> In general, kexec alloc pages from buddy system, it cannot exceed >>>> the physical address in the system. >>>> >>>> The patch just remove this unnecessary code, no functional change. >>> On 32bit systems with highmem support kexec can very easily receive a >>> page from the buddy allocator that can exceed 4GiB. This doesn't show >>> up on 64bit systems as typically the memory limits are less than the >>> address space. But this code is very necessary on some systems and >>> removing it is not ok. >>> >>> Nacked-by: "Eric W. Biederman" <ebiederm at xmission.com> >>> >> This viewpoint is as opposed to me, 32bit systems architectural decide it can not >> access exceed 4GiB whether the highmem or not. but there is one exception, >> when PAE enable, its physical address should be extended to 36, new paging mechanism >> established for it. therefore, the page from the buddy allocator >> can exceed 4GiB. > Exactly. And I was dealing with PAE systems in 2001 or so with > 4GiB > of RAM. Which is where the unusable_pages work comes from. > > Other architectures such as ARM also followed a similar path, so > it isn't just x86 that has 32bit systems with > 32 address lines. > >> moreover, on 32bit systems I can not understand why KEXEC_SOURCE_MEMORY_LIMIT >> is defined to -1UL. therefore, kimge_aloc_page allocate page will always add to unusable_pages. > -1UL is a short way of writing 0xffffffffUL Which is as close as you > can get to writing 0x100000000UL in 32bits. > > kimage_alloc_page won't always add to unusable_pages as there is memory > below 4GiB but it isn't easily found so there may temporarily be a > memory shortage, as it allocates it's way there. Unfortunately whenever > I have looked there are memory zones that line up with the memory the > kexec is looking for. So it does a little bit of a weird dance to get > the memory it needs and to discard the memory it can't use. > > Eric > Thanks , I get it.
