linux-kernel - Re: [PATCH v19 2/7] crash: add generic infrastructure for crash hotplug support

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Message-ID: <ZBQtAYJpC+h9weUD@MiWiFi-R3L-srv>
Date:   Fri, 17 Mar 2023 17:04:01 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Eric DeVolder <eric.devolder@...cle.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, vgoyal@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org, david@...hat.com,
        sourabhjain@...ux.ibm.com, konrad.wilk@...cle.com,
        boris.ostrovsky@...cle.com
Subject: Re: [PATCH v19 2/7] crash: add generic infrastructure for crash
 hotplug support

On 03/16/23 at 09:44am, Eric DeVolder wrote:
> 
> 
> On 3/16/23 05:11, Baoquan He wrote:
> > On 03/06/23 at 11:22am, Eric DeVolder wrote:
> > ......
> > > +static void crash_handle_hotplug_event(unsigned int hp_action, unsigned int cpu)
> > > +{
> > > +	/* Obtain lock while changing crash information */
> > > +	if (kexec_trylock()) {
> > > +
> > > +		/* Check kdump is loaded */
> > > +		if (kexec_crash_image) {
> > > +			struct kimage *image = kexec_crash_image;
> > > +
> > > +			if (hp_action == KEXEC_CRASH_HP_ADD_CPU ||
> > > +				hp_action == KEXEC_CRASH_HP_REMOVE_CPU)
> > > +				pr_debug("hp_action %u, cpu %u\n", hp_action, cpu);
> > > +			else
> > > +				pr_debug("hp_action %u\n", hp_action);
> > > +
> > > +			/*
> > > +			 * When the struct kimage is allocated, the elfcorehdr_index
> > > +			 * is set to -1. Find the segment containing the elfcorehdr,
> > > +			 * if not already found. This works for both the kexec_load
> > > +			 * and kexec_file_load paths.
> > > +			 */
> > > +			if (image->elfcorehdr_index < 0) {
> > > +				unsigned long mem;
> > > +				unsigned char *ptr;
> > > +				unsigned int n;
> > > +
> > > +				for (n = 0; n < image->nr_segments; n++) {
> > > +					mem = image->segment[n].mem;
> > > +					ptr = kmap_local_page(pfn_to_page(mem >> PAGE_SHIFT));
> > > +					if (ptr) {
> > > +						/* The segment containing elfcorehdr */
> > > +						if (memcmp(ptr, ELFMAG, SELFMAG) == 0) {
> > > +							image->elfcorehdr_index = (int)n;
> > > +						}
> > > +						kunmap_local(ptr);
> > > +					}
> > > +				}
> > > +			}
> > > +
> > > +			if (image->elfcorehdr_index < 0) {
> > > +				pr_err("unable to locate elfcorehdr segment");
> > > +				goto out;
> > > +			}
> > > +
> > > +			/* Needed in order for the segments to be updated */
> > > +			arch_kexec_unprotect_crashkres();
> > > +
> > > +			/* Differentiate between normal load and hotplug update */
> > > +			image->hp_action = hp_action;
> > > +
> > > +			/* Now invoke arch-specific update handler */
> > > +			arch_crash_handle_hotplug_event(image);
> > > +
> > > +			/* No longer handling a hotplug event */
> > > +			image->hp_action = KEXEC_CRASH_HP_NONE;
> > > +			image->elfcorehdr_updated = true;
> > 
> > It's good to initialize the image->hp_action here, however where do
> > you check it? Do you plan to add some check somewhere?
> 
> Hi Baoquan,
> The hp_action member is initialized to 0 in do_image_alloc_init(). I've
> mapped KEXEC_CRASH_HP_NONE onto 0 on purpose.
> 
> But the use of image->hp_action = KEXEC_CRASH_HP_NONE is to actually
> delineate that a hotplug event handling has completed. You can see
> imae->hp_action set to hp_action to capture what the triggering event
> was, as passed into this function.
> 
> I will go ahead and set image->hp_action = KEXEC_CRASH_HP_NONE; explicitly
> in do_kimage_alloc_init(), as that is done for the other crash hotplug members.

Yeah, setting image->hp_action = KEXEC_CRASH_HP_NONE in
do_kimage_alloc_init() will make code clearer. While I am wondering if
we don't initialie image->hp_action to KEXEC_CRASH_HP_NONE, and don't
set image->hp_action to KEXEC_CRASH_HP_NONE to actually delineate that a
hotplug event handling has completed, what will happen?

I mean you set image->hp_action to KEXEC_CRASH_HP_NONE explicitly, where
do you check if it should not be KEXEC_CRASH_HP_NONE? In
crash_handle_hotplug_event(), we took __kexec_lock and assign the passed
hp_action anyway.