linux-kernel - Re: [PATCH v3 2/4] hugetlbfs: move hugepagesz= parsing to arch independent code

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Message-ID: <7583dfcc-62d8-2a54-6eef-bcb4e01129b3@gmail.com>
Date:   Mon, 27 Apr 2020 10:34:36 +0530
From:   Sandipan Das <sandipan.osd@...il.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-doc@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "David S . Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Longpeng <longpeng2@...wei.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Randy Dunlap <rdunlap@...radead.org>,
        Mina Almasry <almasrymina@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 2/4] hugetlbfs: move hugepagesz= parsing to arch
 independent code

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