linux-kernel - Re: [PATCH 1/2] vmcore: allow alternate dump capturing methods to export vmcore without is_kdump

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Date:   Tue, 5 Sep 2023 10:30:32 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Hari Bathini <hbathini@...ux.ibm.com>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Kexec-ml <kexec@...ts.infradead.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
        Dave Young <dyoung@...hat.com>
Subject: Re: [PATCH 1/2] vmcore: allow alternate dump capturing methods to
 export vmcore without is_kdump_kernel()

On 09/04/23 at 08:04pm, Hari Bathini wrote:
> Hi Baoquan,
> 
> Thanks for the review...
> 
> On 03/09/23 9:06 am, Baoquan He wrote:
> > Hi Hari,
> > 
> > On 09/02/23 at 12:34am, Hari Bathini wrote:
> > > Currently, is_kdump_kernel() returns true when elfcorehdr_addr is set.
> > > While elfcorehdr_addr is set for kexec based kernel dump mechanism,
> > > alternate dump capturing methods like fadump [1] also set it to export
> > > the vmcore. is_kdump_kernel() is used to restrict resources in crash
> > > dump capture kernel but such restrictions may not be desirable for
> > > fadump. Allow is_kdump_kernel() to be defined differently for such
> > > scenarios. With this, is_kdump_kernel() could be false while vmcore
> > > is usable. So, introduce is_crashdump_kernel() to return true when
> > > elfcorehdr_addr is set and use it for vmcore related checks.
> > 
> > I got what is done in these two patches, but didn't get why they need be
> > done. vmcore_unusable()/is_vmcore_usable() are only unitilized in ia64.
> > Why do you care if it's is_crashdump_kernel() or is_kdump_kernel()?
> > If you want to override the generic is_kdump_kernel() with powerpc's own
> > is_kdump_kernel(), your below change is enough to allow you to do that.
> > I can't see why is_crashdump_kernel() is needed. Could you explain that
> > specifically?
> 
> You mean to just remove is_kdump_kernel() check in is_vmcore_usable() &
> vmcore_unusable() functions? Replaced generic is_crashdump_kernel()
> function instead, that returns true for any dump capturing method,
> irrespective of whether is_kdump_kernel() returns true or false.
> For fadump case, is_kdump_kernel() will return false after patch 2/2.

OK, I could understand what you want to achieve. You want to make
is_kdump_kernel() only return true for kdump, while is_vmcore_usable()
returns true for both kdump and fadump.

IIUC, can we change as below? It could make code clearer and more
straightforward. I don't think adding another is_crashdump_kernel()
is a good idea, that would be a torture for non-powerpc people reading
code when they need differentiate between kdump and crashdump.

diff --git a/include/linux/crash_dump.h b/include/linux/crash_dump.h
index 0f3a656293b0..102a8b710b38 100644
--- a/include/linux/crash_dump.h
+++ b/include/linux/crash_dump.h
@@ -50,6 +50,7 @@ void vmcore_cleanup(void);
 #define vmcore_elf64_check_arch(x) (elf_check_arch(x) || vmcore_elf_check_arch_cross(x))
 #endif
 
+#ifndef is_kdump_active
 /*
  * is_kdump_kernel() checks whether this kernel is booting after a panic of
  * previous kernel or not. This is determined by checking if previous kernel
@@ -64,6 +65,14 @@ static inline bool is_kdump_kernel(void)
 {
 	return elfcorehdr_addr != ELFCORE_ADDR_MAX;
 }
+#endif
+
+#ifndef is_fadump_active
+static inline bool is_fadump_active(void)
+{
+	return false;
+}
+#endif
 
 /* is_vmcore_usable() checks if the kernel is booting after a panic and
  * the vmcore region is usable.
@@ -75,7 +84,8 @@ static inline bool is_kdump_kernel(void)
 
 static inline int is_vmcore_usable(void)
 {
-	return is_kdump_kernel() && elfcorehdr_addr != ELFCORE_ADDR_ERR ? 1 : 0;
+	return (is_kdump_kernel() || is_fadump_active())
+		&& elfcorehdr_addr != ELFCORE_ADDR_ERR ? 1 : 0;
 }
 
 /* vmcore_unusable() marks the vmcore as unusable,
@@ -84,7 +94,7 @@ static inline int is_vmcore_usable(void)
 
 static inline void vmcore_unusable(void)
 {
-	if (is_kdump_kernel())
+	if (is_kdump_kernel() || is_fadump_active())
 		elfcorehdr_addr = ELFCORE_ADDR_ERR;
 }