Re: [PATCH v15 2/5] crash: add a new kexec flag for hotplug support

[Sourabh Jain <sourabhjain@xxxxxxxxxxxxx>]

Hello Baoquan,

On 05/02/24 08:40, Baoquan He wrote:
> Hi Sourabh,
>
> Thanks for the great work. There are some concerns, please see inline
> comments.

Thank you :)

> On 01/11/24 at 04:21pm, Sourabh Jain wrote:
> ......
> > Now, if the kexec tool sends KEXEC_CRASH_HOTPLUG_SUPPORT kexec flag to
> > the kernel, it indicates to the kernel that all the required kexec
> > segment is skipped from SHA calculation and it is safe to update kdump
> > image loaded using the kexec_load syscall.
> So finally you add a new KEXEC_CRASH_HOTPLUG_SUPPORT flag, that's fine.
>
> ......
> > diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
> > index 9bb6607e864e..e791129fdf6c 100644
> > --- a/arch/x86/include/asm/kexec.h
> > +++ b/arch/x86/include/asm/kexec.h
> > @@ -211,6 +211,9 @@ extern void kdump_nmi_shootdown_cpus(void);
> >   void arch_crash_handle_hotplug_event(struct kimage *image, void *arg);
> >   #define arch_crash_handle_hotplug_event arch_crash_handle_hotplug_event
> >   
> > +int arch_crash_hotplug_support(struct kimage *image, unsigned long kexec_flags);
> > +#define arch_crash_hotplug_support arch_crash_hotplug_support
> > +
> >   #ifdef CONFIG_HOTPLUG_CPU
> >   int arch_crash_hotplug_cpu_support(void);
> >   #define crash_hotplug_cpu_support arch_crash_hotplug_cpu_support
> Then crash_hotplug_cpu_support is not needed any more on x86_64, and
> crash_hotplug_memory_support(), if you remove their implementation in
> arch/x86/kernel/crash.c, won't it cause building warning or error on x86?

Yeah, crash_hotplug_cpu_support and crash_hotplug_memory_support are
no longer required. My bad, I forgot to remove them.

> > diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> > index 44744e9c68ec..293b54bff706 100644
> > --- a/arch/x86/kernel/crash.c
> > +++ b/arch/x86/kernel/crash.c
> > @@ -398,20 +398,16 @@ int crash_load_segments(struct kimage *image)
> >   #undef pr_fmt
> >   #define pr_fmt(fmt) "crash hp: " fmt
> >   
> > -/* These functions provide the value for the sysfs crash_hotplug nodes */
> > -#ifdef CONFIG_HOTPLUG_CPU
> > -int arch_crash_hotplug_cpu_support(void)
> > +int arch_crash_hotplug_support(struct kimage *image, unsigned long kexec_flags)
> >   {
> > -	return crash_check_update_elfcorehdr();
> > -}
> > -#endif
> >   
> > -#ifdef CONFIG_MEMORY_HOTPLUG
> > -int arch_crash_hotplug_memory_support(void)
> > -{
> > -	return crash_check_update_elfcorehdr();
> > -}
> > +#ifdef CONFIG_KEXEC_FILE
> > +	if (image->file_mode)
> > +		return 1;
> >   #endif
> > +	return (kexec_flags & KEXEC_UPDATE_ELFCOREHDR ||
> > +		kexec_flags & KEXEC_CRASH_HOTPLUG_SUPPORT);
> Do we need add some document to tell why there are two kexec flags on
> x86_64, except of checking this patch log?

Sure I will add a comment about it.

> > +}
> >   
> >   unsigned int arch_crash_get_elfcorehdr_size(void)
> >   {
> > diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> > index 548491de818e..2f411ddfbd8b 100644
> > --- a/drivers/base/cpu.c
> > +++ b/drivers/base/cpu.c
> > @@ -306,7 +306,7 @@ static ssize_t crash_hotplug_show(struct device *dev,
> >   				     struct device_attribute *attr,
> >   				     char *buf)
> >   {
> > -	return sysfs_emit(buf, "%d\n", crash_hotplug_cpu_support());
> > +	return sysfs_emit(buf, "%d\n", crash_check_hotplug_support());
> >   }
> >   static DEVICE_ATTR_ADMIN_RO(crash_hotplug);
> >   #endif
> > diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> > index 8a13babd826c..e70ab1d3428e 100644
> > --- a/drivers/base/memory.c
> > +++ b/drivers/base/memory.c
> > @@ -514,7 +514,7 @@ static DEVICE_ATTR_RW(auto_online_blocks);
> >   static ssize_t crash_hotplug_show(struct device *dev,
> >   				       struct device_attribute *attr, char *buf)
> >   {
> > -	return sysfs_emit(buf, "%d\n", crash_hotplug_memory_support());
> > +	return sysfs_emit(buf, "%d\n", crash_check_hotplug_support());
> >   }
> >   static DEVICE_ATTR_RO(crash_hotplug);
> >   #endif
> > diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> > index 802052d9c64b..7880d74dc5c4 100644
> > --- a/include/linux/kexec.h
> > +++ b/include/linux/kexec.h
> > @@ -317,8 +317,8 @@ struct kimage {
> >   	/* If set, we are using file mode kexec syscall */
> >   	unsigned int file_mode:1;
> >   #ifdef CONFIG_CRASH_HOTPLUG
> > -	/* If set, allow changes to elfcorehdr of kexec_load'd image */
> > -	unsigned int update_elfcorehdr:1;
> > +	/* If set, allow changes to kexec segments of kexec_load'd image */
> The code comment doesn't reflect the usage of the flag.
I should have updated the comment to indicate that this flag is for both 
system calls.
More comments below.

> You set it too
> when it's kexec_file_load. Speaking of this, I do wonder why you need
> set it too for kexec_file_load,
If we do this one can just access image->hotplug_support to find hotplug
support for currently loaded kdump image without bothering about which
system call was used to load the kdump image.

> and why we have
> arch_crash_hotplug_support(), then crash_check_hotplug_support() both of
> which have the same effect.

arch_crash_hotplug_support(): This function processes the kexec flags 
and finds the
hotplug support for the kdump image. Based on the return value of this 
function,
the image->hotplug_support attribute is set.

Now, once the kdump image is loaded, we no longer have access to the 
kexec flags.
Therefore, crash_check_hotplug_support simply returns the value of 
image->hotplug_support
when user space accesses the following sysfs files: 
/sys/devices/system/[cpu|memory]/crash_hotplug.

To keep things simple, I have introduced two functions: One function 
processes the kexec flags
and determines the hotplug support for the image being loaded. And other 
function simply
accesses image->hotplug_support and advertises CPU/Memory hotplug 
support to userspace.

Let me know you opinion.

Thanks for reviewing the patch.

- Sourabh Jain
> > +	unsigned int hotplug_support:1;
> >   #endif
> >   
> >   #ifdef ARCH_HAS_KIMAGE_ARCH
> > @@ -396,9 +396,10 @@ bool kexec_load_permitted(int kexec_image_type);
> >   
> >   /* List of defined/legal kexec flags */
> >   #ifndef CONFIG_KEXEC_JUMP
> > -#define KEXEC_FLAGS    (KEXEC_ON_CRASH | KEXEC_UPDATE_ELFCOREHDR)
> > +#define KEXEC_FLAGS    (KEXEC_ON_CRASH | KEXEC_UPDATE_ELFCOREHDR | KEXEC_CRASH_HOTPLUG_SUPPORT)
> >   #else
> > -#define KEXEC_FLAGS    (KEXEC_ON_CRASH | KEXEC_PRESERVE_CONTEXT | KEXEC_UPDATE_ELFCOREHDR)
> > +#define KEXEC_FLAGS    (KEXEC_ON_CRASH | KEXEC_PRESERVE_CONTEXT | KEXEC_UPDATE_ELFCOREHDR | \
> > +			KEXEC_CRASH_HOTPLUG_SUPPORT)
> >   #endif
> >   
> >   /* List of defined/legal kexec file flags */
> > @@ -486,14 +487,18 @@ static inline void arch_kexec_pre_free_pages(void *vaddr, unsigned int pages) {
> >   static inline void arch_crash_handle_hotplug_event(struct kimage *image, void *arg) { }
> >   #endif
> >   
> > -int crash_check_update_elfcorehdr(void);
> > +int crash_check_hotplug_support(void);
> >   
> > -#ifndef crash_hotplug_cpu_support
> > -static inline int crash_hotplug_cpu_support(void) { return 0; }
> > -#endif
> > +#ifndef arch_crash_hotplug_support
> > +static inline int arch_crash_hotplug_support(struct kimage *image, unsigned long kexec_flags)
> > +{
> >   
> > -#ifndef crash_hotplug_memory_support
> > -static inline int crash_hotplug_memory_support(void) { return 0; }
> > +#ifdef CONFIG_KEXEC_FILE
> > +	if (image->file_mode)
> > +		return 1;
> > +#endif
> > +	return kexec_flags & KEXEC_CRASH_HOTPLUG_SUPPORT;
> > +}
> >   #endif
> >   
> >   #ifndef crash_get_elfcorehdr_size
> > diff --git a/include/uapi/linux/kexec.h b/include/uapi/linux/kexec.h
> > index c17bb096ea68..5ae1741ea8ea 100644
> > --- a/include/uapi/linux/kexec.h
> > +++ b/include/uapi/linux/kexec.h
> > @@ -13,6 +13,7 @@
> >   #define KEXEC_ON_CRASH		0x00000001
> >   #define KEXEC_PRESERVE_CONTEXT	0x00000002
> >   #define KEXEC_UPDATE_ELFCOREHDR	0x00000004
> > +#define KEXEC_CRASH_HOTPLUG_SUPPORT 0x00000008
> >   #define KEXEC_ARCH_MASK		0xffff0000
> >   
> >   /*
> > diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> > index ab1c8e79759d..111548ad03e9 100644
> > --- a/kernel/crash_core.c
> > +++ b/kernel/crash_core.c
> > @@ -876,7 +876,7 @@ DEFINE_MUTEX(__crash_hotplug_lock);
> >    * It reflects the kernel's ability/permission to update the crash
> >    * elfcorehdr directly.
> >    */
> > -int crash_check_update_elfcorehdr(void)
> > +int crash_check_hotplug_support(void)
> >   {
> >   	int rc = 0;
> >   
> > @@ -888,10 +888,7 @@ int crash_check_update_elfcorehdr(void)
> >   		return 0;
> >   	}
> >   	if (kexec_crash_image) {
> > -		if (kexec_crash_image->file_mode)
> > -			rc = 1;
> > -		else
> > -			rc = kexec_crash_image->update_elfcorehdr;
> > +		rc = kexec_crash_image->hotplug_support;
> >   	}
> >   	/* Release lock now that update complete */
> >   	kexec_unlock();
> > @@ -932,8 +929,8 @@ static void crash_handle_hotplug_event(unsigned int hp_action, unsigned int cpu,
> >   
> >   	image = kexec_crash_image;
> >   
> > -	/* Check that updating elfcorehdr is permitted */
> > -	if (!(image->file_mode || image->update_elfcorehdr))
> > +	/* Check that kexec segments update is permitted */
> > +	if (!image->hotplug_support)
> >   		goto out;
> >   
> >   	if (hp_action == KEXEC_CRASH_HP_ADD_CPU ||
> > diff --git a/kernel/kexec.c b/kernel/kexec.c
> > index 8f35a5a42af8..9dc5b7ed5b73 100644
> > --- a/kernel/kexec.c
> > +++ b/kernel/kexec.c
> > @@ -130,8 +130,8 @@ static int do_kexec_load(unsigned long entry, unsigned long nr_segments,
> >   		image->preserve_context = 1;
> >   
> >   #ifdef CONFIG_CRASH_HOTPLUG
> > -	if (flags & KEXEC_UPDATE_ELFCOREHDR)
> > -		image->update_elfcorehdr = 1;
> > +	if ((flags & KEXEC_ON_CRASH) && arch_crash_hotplug_support(image, flags))
> > +		image->hotplug_support = 1;
> >   #endif
> >   
> >   	ret = machine_kexec_prepare(image);
> > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> > index bef2f6f2571b..4dddf9264a69 100644
> > --- a/kernel/kexec_file.c
> > +++ b/kernel/kexec_file.c
> > @@ -373,6 +373,11 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
> >   	if (ret)
> >   		goto out;
> >   
> > +#ifdef CONFIG_CRASH_HOTPLUG
> > +	if ((flags & KEXEC_FILE_ON_CRASH) && arch_crash_hotplug_support(image, flags))
> > +		image->hotplug_support = 1;
> > +#endif
> > +
> >   	ret = machine_kexec_prepare(image);
> >   	if (ret)
> >   		goto out;
> > --
> > 2.41.0


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