linux-kernel - Re: [PATCH 2/6] thp: optimize away unnecessary page table locking

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Message-ID: <4F104A51.2000701@ah.jp.nec.com>
Date:	Fri, 13 Jan 2012 10:14:25 -0500
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Hillf Danton <dhillf@...il.com>
CC:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/6] thp: optimize away unnecessary page table locking

Hi Hillf,

(1/13/2012 7:04), Hillf Danton wrote:
[...]
>> +/*
>> + * Returns 1 if a given pmd is mapping a thp and stable (not under splitting.)
>> + * Returns 0 otherwise. Note that if it returns 1, this routine returns without
>> + * unlocking page table locks. So callers must unlock them.
>> + */
>> +int pmd_trans_huge_stable(pmd_t *pmd, struct vm_area_struct *vma)
>> +{
>> +       VM_BUG_ON(!rwsem_is_locked(&vma->vm_mm->mmap_sem));
>> +
>> +       if (!pmd_trans_huge(*pmd))
>> +               return 0;
>> +
>> +       spin_lock(&vma->vm_mm->page_table_lock);
>> +       if (likely(pmd_trans_huge(*pmd))) {
>> +               if (pmd_trans_splitting(*pmd)) {
>> +                       spin_unlock(&vma->vm_mm->page_table_lock);
>> +                       wait_split_huge_page(vma->anon_vma, pmd);
>> +                       return 0;
>> +               } else {
> 
>                             spin_unlock(&vma->vm_mm->page_table_lock);     yes?

No. Unlocking is supposed to be done by the caller as commented.

Thanks,
Naoya

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

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