On 07/23/18 at 04:34pm, Michal Hocko wrote:
> On Thu 19-07-18 23:17:53, Baoquan He wrote:
> > Kexec has been a formal feature in our distro, and customers owning
> > those kind of very large machine can make use of this feature to speed
> > up the reboot process. On uefi machine, the kexec_file loading will
> > search place to put kernel under 4G from top to down. As we know, the
> > 1st 4G space is DMA32 ZONE, dma, pci mmcfg, bios etc all try to consume
> > it. It may have possibility to not be able to find a usable space for
> > kernel/initrd. From the top down of the whole memory space, we don't
> > have this worry.
>
> I do not have the full context here but let me note that you should be
> careful when doing top-down reservation because you can easily get into
> hotplugable memory and break the hotremove usecase. We even warn when
> this is done. See memblock_find_in_range_node
Kexec read kernel/initrd file into buffer, just search usable positions
for them to do the later copying. You can see below struct kexec_segment,
for the old kexec_load, kernel/initrd are read into user space buffer,
the @buf stores the user space buffer address, @mem stores the position
where kernel/initrd will be put. In kernel, it calls
kimage_load_normal_segment() to copy user space buffer to intermediate
pages which are allocated with flag GFP_KERNEL. These intermediate pages
are recorded as entries, later when user execute "kexec -e" to trigger
kexec jumping, it will do the final copying from the intermediate pages
to the real destination pages which @mem pointed. Because we can't touch
the existed data in 1st kernel when do kexec kernel loading. With my
understanding, GFP_KERNEL will make those intermediate pages be
allocated inside immovable area, it won't impact hotplugging. But the
@mem we searched in the whole system RAM might be lost along with
hotplug. Hence we need do kexec kernel again when hotplug event is
detected.
#define KEXEC_CONTROL_MEMORY_GFP (GFP_KERNEL | __GFP_NORETRY)
struct kexec_segment {
/*
* This pointer can point to user memory if kexec_load() system
* call is used or will point to kernel memory if
* kexec_file_load() system call is used.
*
* Use ->buf when expecting to deal with user memory and use ->kbuf
* when expecting to deal with kernel memory.
*/
union {
void __user *buf;
void *kbuf;
};
size_t bufsz;
unsigned long mem;
size_t memsz;
};
Thanks
Baoquan
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