Re: [PATCH v16 13/25] mm: pagewalk: Don't lock PTEs for walk_page_range_novma()

From: Steven Price <steven.price@arm.com>
To: kbuild test robot <lkp@intel.com>
Cc: "Mark Rutland" <Mark.Rutland@arm.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	linux-mm@kvack.org, "H. Peter Anvin" <hpa@zytor.com>,
	"Will Deacon" <will@kernel.org>,
	"Liang, Kan" <kan.liang@linux.intel.com>,
	x86@kernel.org, "Ingo Molnar" <mingo@redhat.com>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	linux-arm-kernel@lists.infradead.org, kbuild-all@lists.01.org,
	"Ard Biesheuvel" <ard.biesheuvel@linaro.org>,
	linux-kernel@vger.kernel.org, "James Morse" <james.morse@arm.com>,
	"Andrew Morton" <akpm@linux-foundation.org>
Subject: Re: [PATCH v16 13/25] mm: pagewalk: Don't lock PTEs for walk_page_range_novma()
Date: Wed, 11 Dec 2019 15:54:06 +0000	[thread overview]
Message-ID: <e0fd5594-fb4e-9ead-e582-544f47cb1f8b@arm.com> (raw)
In-Reply-To: <201912101842.KIXI4yCg%lkp@intel.com>

On 10/12/2019 11:23, kbuild test robot wrote:
> Hi Steven,
> 
> I love your patch! Perhaps something to improve:
> 
> [auto build test WARNING on linus/master]
> [also build test WARNING on v5.5-rc1 next-20191209]
> [cannot apply to arm64/for-next/core tip/x86/mm]
> [if your patch is applied to the wrong git tree, please drop us a note to help
> improve the system. BTW, we also suggest to use '--base' option to specify the
> base tree in git format-patch, please see https://stackoverflow.com/a/37406982]
> 
> url:    https://github.com/0day-ci/linux/commits/Steven-Price/Generic-page-walk-and-ptdump/20191208-035831
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git ad910e36da4ca3a1bd436989f632d062dda0c921
> reproduce:
>         # apt-get install sparse
>         # sparse version: v0.6.1-101-g82dee2e-dirty
>         make ARCH=x86_64 allmodconfig
>         make C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__'
> 
> If you fix the issue, kindly add following tag
> Reported-by: kbuild test robot <lkp@intel.com>
> 
> 
> sparse warnings: (new ones prefixed by >>)
> 
>>> include/linux/spinlock.h:378:9: sparse: sparse: context imbalance in 'walk_pte_range' - unexpected unlock

I believe this is a false positive (although the trace here is useless).
This patch adds a conditional lock/unlock:

pte = walk->no_vma ? pte_offset_map(pmd, addr) :
		     pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
...
if (!walk->no_vma)
	spin_unlock(ptl);
pte_unmap(pte);

I'm not sure how to match sparse happy about that. Is the only option to
have two versions of the walk_pte_range() function? One which takes the
lock and one which doesn't.

Steve

> vim +/walk_pte_range +378 include/linux/spinlock.h
> 
> c2f21ce2e31286 Thomas Gleixner 2009-12-02  375  
> 3490565b633c70 Denys Vlasenko  2015-07-13  376  static __always_inline void spin_unlock(spinlock_t *lock)
> c2f21ce2e31286 Thomas Gleixner 2009-12-02  377  {
> c2f21ce2e31286 Thomas Gleixner 2009-12-02 @378  	raw_spin_unlock(&lock->rlock);
> c2f21ce2e31286 Thomas Gleixner 2009-12-02  379  }
> c2f21ce2e31286 Thomas Gleixner 2009-12-02  380  
> 
> :::::: The code at line 378 was first introduced by commit
> :::::: c2f21ce2e31286a0a32f8da0a7856e9ca1122ef3 locking: Implement new raw_spinlock
> 
> :::::: TO: Thomas Gleixner <tglx@linutronix.de>
> :::::: CC: Thomas Gleixner <tglx@linutronix.de>
> 
> ---
> 0-DAY kernel test infrastructure                 Open Source Technology Center
> https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org Intel Corporation
> 
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