From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from out01.mta.xmission.com ([166.70.13.231]) by bombadil.infradead.org with esmtps (Exim 4.85_2 #1 (Red Hat Linux)) id 1bNCai-0003j7-83 for kexec@lists.infradead.org; Wed, 13 Jul 2016 05:20:12 +0000 From: ebiederm@xmission.com (Eric W. Biederman) References: <1468299403-27954-1-git-send-email-zhongjiang@huawei.com> <87poqi3muo.fsf@x220.int.ebiederm.org> <5785BEA6.2060404@huawei.com> Date: Wed, 13 Jul 2016 00:07:18 -0500 In-Reply-To: <5785BEA6.2060404@huawei.com> (zhong jiang's message of "Wed, 13 Jul 2016 12:08:06 +0800") Message-ID: <87lh16unw9.fsf@x220.int.ebiederm.org> MIME-Version: 1.0 Subject: Re: [PATCH 1/2] kexec: remove unnecessary unusable_pages List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "kexec" Errors-To: kexec-bounces+dwmw2=infradead.org@lists.infradead.org To: zhong jiang Cc: dyoung@redhat.com, kexec@lists.infradead.org, linux-mm@kvack.org, horms@verge.net.au, akpm@linux-foundation.org, yinghai@kernel.org, vgoyal@redhat.com zhong jiang writes: > On 2016/7/12 23:19, Eric W. Biederman wrote: >> zhongjiang writes: >> >>> From: zhong jiang >>> >>> 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" >> > 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 _______________________________________________ kexec mailing list kexec@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kexec