Messages in this thread | | | Date | Wed, 12 Aug 2009 10:08:04 +0800 | From | Amerigo Wang <> | Subject | Re: [RFC Patch 2/2] kexec: allow to shrink reserved memory |
| |
Eric W. Biederman wrote: > Amerigo Wang <amwang@redhat.com> writes: > > >> Eric W. Biederman wrote: >> >>> Amerigo Wang <amwang@redhat.com> 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...
| |