Re: [RFC Patch 2/2] kexec: allow to shrink reserved memory

Neil Horman
2009-08-11T10:46:06Z

On Tue, Aug 11, 2009 at 06:39:32AM -0400, Amerigo Wang wrote:
>
> 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
>
> Signed-off-by: WANG Cong <amwang@redhat.com>
> Cc: Neil Horman <nhorman@redhat.com>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: Andi Kleen <andi@firstfloor.org>
>

Since the reserved area is also used for heap in the new kernel, isn't this
mechanism going to guarantee a non-bootable kernel. It seems like it shrinks
the reserved area to the size of the image, leaving no additional memory for
heap allocations during the kernels boot. Or am I missing something?

Neil

> ---
> Index: linux-2.6/include/linux/kexec.h
> =================================> --- linux-2.6.orig/include/linux/kexec.h
> +++ linux-2.6/include/linux/kexec.h
> @@ -158,6 +158,8 @@ unsigned long paddr_vmcoreinfo_note(void
>
> extern struct kimage *kexec_image;
> extern struct kimage *kexec_crash_image;
> +extern struct resource *kexec_res;
> +extern struct resource *kexec_free_res;
>
> #ifndef kexec_flush_icache_page
> #define kexec_flush_icache_page(page)
> @@ -206,6 +208,7 @@ extern size_t vmcoreinfo_max_size;
>
> int __init parse_crashkernel(char *cmdline, unsigned long long system_ram,
> unsigned long long *crash_size, unsigned long long *crash_base);
> +int shrink_crash_memory(unsigned long start, unsigned long end);
>
> #else /* !CONFIG_KEXEC */
> struct pt_regs;
> Index: linux-2.6/kernel/kexec.c
> =================================> --- linux-2.6.orig/kernel/kexec.c
> +++ linux-2.6/kernel/kexec.c
> @@ -1130,6 +1130,51 @@ void crash_kexec(struct pt_regs *regs)
> }
> }
>
> +int shrink_crash_memory(unsigned long start, unsigned long end)
> +{
> + struct page **pages;
> + int ret = 0;
> + int npages, i;
> + unsigned long addr;
> + void *vaddr;
> +
> + if (!mutex_trylock(&kexec_mutex))
> + return -EBUSY;
> +
> + if (!kexec_free_res) {
> + ret = -ENOENT;
> + goto unlock;
> + }
> +
> + start = roundup(start, PAGE_SIZE);
> + end = roundup(end, PAGE_SIZE) - 1;
> + npages = (end + 1 - start ) / PAGE_SIZE;
> +
> + pages = kmalloc(sizeof(struct page *) * npages, GFP_KERNEL);
> + if (!pages) {
> + ret = -ENOMEM;
> + goto unlock;
> + }
> + for (i = 0; i < npages; i++) {
> + addr = start + i * PAGE_SIZE;
> + pages[i] = virt_to_page(addr);
> + }
> +
> + vaddr = vm_map_ram(pages, npages, 0, PAGE_KERNEL);
> + if (!vaddr) {
> + ret = -ENOMEM;
> + goto free;
> + }
> + kexec_free_res->end = start - 1;
> + crashk_res.end = start - 1;
> +
> +free:
> + kfree(pages);
> +unlock:
> + mutex_unlock(&kexec_mutex);
> + return ret;
> +}
> +
> static u32 *append_elf_note(u32 *buf, char *name, unsigned type, void *data,
> size_t data_len)
> {
> Index: linux-2.6/kernel/ksysfs.c
> =================================> --- linux-2.6.orig/kernel/ksysfs.c
> +++ linux-2.6/kernel/ksysfs.c
> @@ -100,6 +100,32 @@ static ssize_t kexec_crash_loaded_show(s
> }
> KERNEL_ATTR_RO(kexec_crash_loaded);
>
> +static ssize_t kexec_crash_size_show(struct kobject *kobj,
> + struct kobj_attribute *attr, char *buf)
> +{
> + return sprintf(buf, "%llu\n", crashk_res.end - crashk_res.start + 1);
> +}
> +static ssize_t kexec_crash_size_store(struct kobject *kobj,
> + struct kobj_attribute *attr,
> + const char *buf, size_t count)
> +{
> + unsigned long cnt;
> + int ret;
> +
> + if (!kexec_res || !kexec_free_res)
> + return -ENOENT;
> + cnt = simple_strtoul(buf, NULL, 10);
> + if (cnt < kexec_res->end - kexec_res->start + 1)
> + return -ENOENT;
> + if (cnt > kexec_free_res->end - kexec_res->start + 1)
> + return -ENOENT;
> + cnt -= kexec_res->end - kexec_res->start + 1;
> + ret = shrink_crash_memory(kexec_free_res->start + cnt,
> + kexec_free_res->end);
> + return ret < 0 ? ret : count;
> +}
> +KERNEL_ATTR_RW(kexec_crash_size);
> +
> static ssize_t vmcoreinfo_show(struct kobject *kobj,
> struct kobj_attribute *attr, char *buf)
> {
> @@ -147,6 +173,7 @@ static struct attribute * kernel_attrs[]
> #ifdef CONFIG_KEXEC
> &kexec_loaded_attr.attr,
> &kexec_crash_loaded_attr.attr,
> + &kexec_crash_size_attr.attr,
> &vmcoreinfo_attr.attr,
> #endif
> NULL