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Subject: Re: [PATCH] mm/hmm: fix unused variable warnings
To:     Arnd Bergmann <arnd@arndb.de>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        John Hubbard <jhubbard@nvidia.com>,
        Dan Williams <dan.j.williams@intel.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20190304200026.1140281-1-arnd@arndb.de>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <be817f74-3441-47c1-6958-233d6e1172c4@arm.com>
Date:   Tue, 5 Mar 2019 17:48:07 +0530
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On 03/05/2019 01:30 AM, Arnd Bergmann wrote:
> When CONFIG_HUGETLB_PAGE is disabled, the only use of the variable 'h'
> is compiled out, and the compiler thinks it is unnecessary:
> 
> mm/hmm.c: In function 'hmm_range_snapshot':
> mm/hmm.c:1015:19: error: unused variable 'h' [-Werror=unused-variable]
>     struct hstate *h = hstate_vma(vma);

After doing some Kconfig hacks like (ARCH_WANT_GENERAL_HUGETLB = n) on an
X86 system I got (HUGETLB_PAGE = n and HMM = y) config. But was unable to
hit the build error. Helper is_vm_hugetlb_page() seems to always return
false when HUGETLB_PAGE = n. Would not the compiler remove the entire code
block including the declaration for 'h' ?

#ifdef CONFIG_HUGETLB_PAGE
#include <linux/mm.h>
static inline bool is_vm_hugetlb_page(struct vm_area_struct *vma)
{
        return !!(vma->vm_flags & VM_HUGETLB);
}
#else
static inline bool is_vm_hugetlb_page(struct vm_area_struct *vma)
{
        return false;
}
#endif