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Subject: Re: [PATCH v3 2/4] hugetlbfs: move hugepagesz= parsing to arch
 independent code
To:     Mike Kravetz <mike.kravetz@oracle.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
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        linux-doc@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        "David S . Miller" <davem@davemloft.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Longpeng <longpeng2@huawei.com>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Randy Dunlap <rdunlap@infradead.org>,
        Mina Almasry <almasrymina@google.com>,
        Peter Xu <peterx@redhat.com>,
        Nitesh Narayan Lal <nitesh@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20200417185049.275845-1-mike.kravetz@oracle.com>
 <20200417185049.275845-3-mike.kravetz@oracle.com>
From:   Sandipan Das <sandipan.osd@gmail.com>
Message-ID: <7583dfcc-62d8-2a54-6eef-bcb4e01129b3@gmail.com>
Date:   Mon, 27 Apr 2020 10:34:36 +0530
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Hi Mike,

On 18/04/20 12:20 am, Mike Kravetz wrote:
> Now that architectures provide arch_hugetlb_valid_size(), parsing
> of "hugepagesz=" can be done in architecture independent code.
> Create a single routine to handle hugepagesz= parsing and remove
> all arch specific routines.  We can also remove the interface
> hugetlb_bad_size() as this is no longer used outside arch independent
> code.
> 
> This also provides consistent behavior of hugetlbfs command line
> options.  The hugepagesz= option should only be specified once for
> a specific size, but some architectures allow multiple instances.
> This appears to be more of an oversight when code was added by some
> architectures to set up ALL huge pages sizes.
> 
> [...]
> 
> diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
> index de54d2a37830..2c3fa0a7787b 100644
> --- a/arch/powerpc/mm/hugetlbpage.c
> +++ b/arch/powerpc/mm/hugetlbpage.c
> @@ -589,21 +589,6 @@ static int __init add_huge_page_size(unsigned long long size)
>  	return 0;
>  }
>  
> -static int __init hugepage_setup_sz(char *str)
> -{
> -	unsigned long long size;
> -
> -	size = memparse(str, &str);
> -
> -	if (add_huge_page_size(size) != 0) {
> -		hugetlb_bad_size();
> -		pr_err("Invalid huge page size specified(%llu)\n", size);
> -	}
> -
> -	return 1;
> -}
> -__setup("hugepagesz=", hugepage_setup_sz);
> -
> [...]

This isn't working as expected on powerpc64.

  [    0.000000] Kernel command line: root=UUID=dc7b49cf-95a2-4996-8e7d-7c64ddc7a6ff hugepagesz=16G hugepages=2 
  [    0.000000] HugeTLB: huge pages not supported, ignoring hugepagesz = 16G
  [    0.000000] HugeTLB: huge pages not supported, ignoring hugepages = 2
  [    0.284177] HugeTLB registered 16.0 MiB page size, pre-allocated 0 pages
  [    0.284182] HugeTLB registered 16.0 GiB page size, pre-allocated 0 pages
  [    2.585062]     hugepagesz=16G
  [    2.585063]     hugepages=2

The "huge pages not supported" messages are under a !hugepages_supported()
condition which checks if HPAGE_SHIFT is non-zero. On powerpc64, HPAGE_SHIFT
comes from the hpage_shift variable. At this point, it is still zero and yet
to be set. Hence the check fails. The reason being hugetlbpage_init_default(),
which sets hpage_shift, it now called after hugepage_setup_sz().


- Sandipan