Re: [PATCH v7 4/4] kexec_file: Load kernel at top of system RAM if required

On 07/23/18 at 04:34pm, Michal Hocko wrote:

On Thu 19-07-18 23:17:53, Baoquan He wrote:

quoted

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