From mboxrd@z Thu Jan 1 00:00:00 1970 From: Amerigo Wang Date: Wed, 12 Aug 2009 02:08:04 +0000 Subject: Re: [RFC Patch 2/2] kexec: allow to shrink reserved memory Message-Id: <4A822404.9030603@redhat.com> List-Id: References: <20090811104144.5154.77871.sendpatchset@localhost.localdomain> <20090811104154.5154.78710.sendpatchset@localhost.localdomain> <4A821A14.1020408@redhat.com> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: "Eric W. Biederman" Cc: linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org, Neil Horman , Andi Kleen , akpm@linux-foundation.org, Ingo Molnar Eric W. Biederman wrote: > Amerigo Wang writes: > > >> Eric W. Biederman wrote: >> >>> Amerigo Wang writes: >>> >>> >>> >>>> This patch implements shrinking the reserved memory for crash kernel, >>>> if it is more than enough. >>>> >>>> For example, if you have already reserved 128M, now you just want 100M, >>>> you can do: >>>> >>>> # echo $((100*1024*1024)) > /sys/kernel/kexec_crash_size >>>> >>>> >>> This patch looks like a reasonable start. >>> >>> However once a crash kernel image is loaded we have already told that >>> image about the memory that is available and what you are doing here >>> will go and stop on the memory that is reserved but not yet used, >>> totally breaking the DMA protections. AKA we know the memory is safe >>> from ongoing DMAs because it has lain fallow since boot up. >>> >>> The only safe thing to do is to reduce the memory size before (possibly >>> just before) we load the crash kernel. Which means we should only >>> be allowed to shrink the size when nothing is loaded, exactly the >>> opposite of what you have implemented. >>> >>> >>> >> Confused, why just loading the crash kernel makes it unsafe? >> DMA should be avoided when reserving that memory during boot, shouldn't it? >> > > Yes. But you are removing the reservation and starting DMA on memory > we have told the crash kernel it can use. > We can modify the info given to the crash kernel. > >> I know I missed the part that freeing memory before loading, but if it is safe >> before loading, how can it be unsafe after that? >> > > We tell the crash kernel when loading it, it can use all of the reserved memory. > Yeah, but we should reload the kernel after shrinking the memory, it is not surprised that doing this is necessary...