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Date: Thu, 22 Dec 2011 21:04:23 +0800
Message-ID: <CAJd=RBDxZbsk8RxFpHUJtsc=fq+=WWGeWvJGJX_SFFGj4AvuHg@mail.gmail.com>
Subject: Re: [PATCH 2/4] thp: optimize away unnecessary page table locking
From: Hillf Danton <dhillf@gmail.com>
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To: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, David Rientjes <rientjes@google.com>, Andi Kleen <andi@firstfloor.org>, Wu Fengguang <fengguang.wu@intel.com>, Andrea Arcangeli <aarcange@redhat.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

On Thu, Dec 22, 2011 at 6:23 AM, Naoya Horiguchi
<n-horiguchi@ah.jp.nec.com> wrote:
> Currently when we check if we can handle thp as it is or we need to
> split it into regular sized pages, we hold page table lock prior to
> check whether a given pmd is mapping thp or not. Because of this,
> when it's not "huge pmd" we suffer from unnecessary lock/unlock overhead.
> To remove it, this patch introduces a optimized check function and
> replace several similar logics with it.
>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: David Rientjes <rientjes@google.com>
> ---
> =C2=A0fs/proc/task_mmu.c =C2=A0 =C2=A0 =C2=A0| =C2=A0 74 ++++++++++------=
------------
> =C2=A0include/linux/huge_mm.h | =C2=A0 =C2=A07 +++
> =C2=A0mm/huge_memory.c =C2=A0 =C2=A0 =C2=A0 =C2=A0| =C2=A0124 +++++++++++=
+++++++++++------------------------
> =C2=A0mm/mremap.c =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 | =C2=A0 =C2=
=A03 +-
> =C2=A04 files changed, 93 insertions(+), 115 deletions(-)
>

[...]

>
> +/*
> + * Returns 1 if a given pmd is mapping a thp and stable (not under split=
ting.)
> + * Returns 0 otherwise. Note that if it returns 1, this routine returns =
without
> + * unlocking page table locks. So callers must unlock them.
> + */
> +int check_and_wait_split_huge_pmd(pmd_t *pmd, struct vm_area_struct *vma=
)
> +{
> + =C2=A0 =C2=A0 =C2=A0 if (!pmd_trans_huge(*pmd))
> + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return 0;
> +
> + =C2=A0 =C2=A0 =C2=A0 spin_lock(&vma->vm_mm->page_table_lock);
> + =C2=A0 =C2=A0 =C2=A0 if (likely(pmd_trans_huge(*pmd))) {
> + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 if (pmd_trans_splittin=
g(*pmd)) {
> + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 spin_unlock(&vma->vm_mm->page_table_lock);
> + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 wait_split_huge_page(vma->anon_vma, pmd);

                            return 0;   yes?

> + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 } else {
> + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 /* Thp mapped by 'pmd' is stable, so we can
> + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0* handle it as it is. */
> + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 return 1;
> + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 }
> + =C2=A0 =C2=A0 =C2=A0 }
> + =C2=A0 =C2=A0 =C2=A0 spin_unlock(&vma->vm_mm->page_table_lock);
> + =C2=A0 =C2=A0 =C2=A0 return 0;
> +}
> +

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