linux-kernel - Re: [tip:x86/asm] x86/asm/entry/64: Remove unused thread

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Date:	Tue, 17 Mar 2015 10:01:37 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	linux-tip-commits@...r.kernel.org, linux-kernel@...r.kernel.org,
	keescook@...omium.org, ast@...mgrid.com, fweisbec@...il.com,
	oleg@...hat.com, tglx@...utronix.de, torvalds@...ux-foundation.org,
	hpa@...or.com, wad@...omium.org, rostedt@...dmis.org
Subject: Re: [tip:x86/asm] x86/asm/entry/64: Remove unused
 thread_struct::usersp

On Tue, Mar 17, 2015 at 09:27:36AM +0100, Ingo Molnar wrote:
> Ok, in any case I'm doing a rebase of the affected commits in
> tip:x86/asm. That's a tree where we don't want to break bisectability,
> and this breakage looks sufficiently mysterious.

Yeah.

And it keeps getting better and better. I added some debug output to the
PF-path to see why exactly we segfault and the guest booted fine! No
segfaults. So add timing to that failure :-(

---
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index ede025fb46f1..54ca8539f345 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -746,6 +746,9 @@ show_signal_msg(struct pt_regs *regs, unsigned long error_code,
 	print_vma_addr(KERN_CONT " in ", regs->ip);
 
 	printk(KERN_CONT "\n");
+
+	dump_stack();
+
 }
 
 static void
@@ -827,8 +830,10 @@ __bad_area(struct pt_regs *regs, unsigned long error_code,
 }
 
 static noinline void
-bad_area(struct pt_regs *regs, unsigned long error_code, unsigned long address)
+bad_area(const char *where, struct pt_regs *regs, unsigned long error_code,
+	 unsigned long address)
 {
+	pr_emerg("%s: %s\n", __func__, where);
 	__bad_area(regs, error_code, address, SEGV_MAPERR);
 }
 
@@ -1189,13 +1194,13 @@ retry:
 
 	vma = find_vma(mm, address);
 	if (unlikely(!vma)) {
-		bad_area(regs, error_code, address);
+		bad_area("!vma", regs, error_code, address);
 		return;
 	}
 	if (likely(vma->vm_start <= address))
 		goto good_area;
 	if (unlikely(!(vma->vm_flags & VM_GROWSDOWN))) {
-		bad_area(regs, error_code, address);
+		bad_area("!VM_GROWSDOWN", regs, error_code, address);
 		return;
 	}
 	if (error_code & PF_USER) {
@@ -1206,12 +1211,12 @@ retry:
 		 * 32 pointers and then decrements %sp by 65535.)
 		 */
 		if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) {
-			bad_area(regs, error_code, address);
+			bad_area("65536", regs, error_code, address);
 			return;
 		}
 	}
 	if (unlikely(expand_stack(vma, address))) {
-		bad_area(regs, error_code, address);
+		bad_area("expand_stack", regs, error_code, address);
 		return;
 	}
 
diff --git a/mm/mmap.c b/mm/mmap.c
index da9990acc08b..47c4321f0d18 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2225,13 +2225,17 @@ int expand_downwards(struct vm_area_struct *vma,
 	 * We must make sure the anon_vma is allocated
 	 * so that the anon_vma locking is not a noop.
 	 */
-	if (unlikely(anon_vma_prepare(vma)))
+	if (unlikely(anon_vma_prepare(vma))) {
+		pr_err("anon_vma_prepare\n");
 		return -ENOMEM;
+	}
 
 	address &= PAGE_MASK;
 	error = security_mmap_addr(address);
-	if (error)
+	if (error) {
+		pr_err("security_mmap_addr\n");
 		return error;
+	}
 
 	vma_lock_anon_vma(vma);
 
@@ -2278,6 +2282,7 @@ int expand_downwards(struct vm_area_struct *vma,
 	vma_unlock_anon_vma(vma);
 	khugepaged_enter_vma_merge(vma, vma->vm_flags);
 	validate_mm(vma->vm_mm);
+	pr_err("validate_mm: %d\n", error);
 	return error;
 }
 
@@ -2326,11 +2331,15 @@ int expand_stack(struct vm_area_struct *vma, unsigned long address)
 {
 	struct vm_area_struct *prev;
 
+	pr_err("%s: address: 0x%lx\n", __func__, address);
+
 	address &= PAGE_MASK;
 	prev = vma->vm_prev;
 	if (prev && prev->vm_end == address) {
-		if (!(prev->vm_flags & VM_GROWSDOWN))
+		if (!(prev->vm_flags & VM_GROWSDOWN)) {
+			pr_err("!(prev->vm_flags & VM_GROWSDOWN)\n");
 			return -ENOMEM;
+		}
 	}
 	return expand_downwards(vma, address);
 }

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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