linux-kernel - Re: [PATCH] mm/hugetlb: remove gigantic page support for HIGHMEM

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Date:   Wed, 11 Jul 2018 14:36:16 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michal Hocko <mhocko@...nel.org>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm/hugetlb: remove gigantic page support for HIGHMEM

On 07/11/2018 01:57 PM, Davidlohr Bueso wrote:
> On Wed, 11 Jul 2018, Mike Kravetz wrote:
> 
>> This reverts commit ee8f248d266e ("hugetlb: add phys addr to struct
>> huge_bootmem_page")
>>
>> At one time powerpc used this field and supporting code. However that
>> was removed with commit 79cc38ded1e1 ("powerpc/mm/hugetlb: Add support
>> for reserving gigantic huge pages via kernel command line").
>>
>> There are no users of this field and supporting code, so remove it.
> 
> Considering the title, don't you wanna also get rid of try_to_free_low()
> and something like the following, which I'm sure can be done fancier, and
> perhaps also thp?

Not really.  The intention is to only remove gigantic huge page support for
HIGHMEN systems.  Non-gigantic huge pages on HIGHMEN systems should still
work/be supported.  So, we do not want to make the config change or get
rid of try_to_free_low().

Actually, I simply wanted to revert the specific patch which enabled gigantic
huge pages on HIGHMEM systems.  I did see that check in try_to_free_low(),

	if (hstate_is_gigantic(h))
		return;

and considered for a minute turning that into a VM_BUG or WARN, but decided
to leave it as is.

Do you think the title should be changed to simply 'revert commit
ee8f248d266e'?

-- 
Mike Kravetz

> diff --git a/fs/Kconfig b/fs/Kconfig
> index ac474a61be37..849da70e35d6 100644
> --- a/fs/Kconfig
> +++ b/fs/Kconfig
> @@ -192,8 +192,8 @@ config TMPFS_XATTR
> 
> config HUGETLBFS
>        bool "HugeTLB file system support"
> -       depends on X86 || IA64 || SPARC64 || (S390 && 64BIT) || \
> -                  SYS_SUPPORTS_HUGETLBFS || BROKEN
> +       depends on !HIGHMEM && (X86 || IA64 || SPARC64 || (S390 && 64BIT) || \
> +                  SYS_SUPPORTS_HUGETLBFS || BROKEN)
>        help
>          hugetlbfs is a filesystem backing for HugeTLB pages, based on
>          ramfs. For architectures that support it, say Y here and read
> 
> Thanks,
> Davidlohr