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SubjectRe: [PATCH 2/6] thp: optimize away unnecessary page table locking
On Fri, 27 Jan 2012 18:02:49 -0500
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>
>
> Changes since v3:
> - Fix likely/unlikely pattern in pmd_trans_huge_stable()
> - Change suffix from _stable to _lock
> - Introduce __pmd_trans_huge_lock() to avoid micro-regression
> - Return 1 when wait_split_huge_page path is taken
>
> Changes since v2:
> - Fix missing "return 0" in "thp under splitting" path
> - Remove unneeded comment
> - Change the name of check function to describe what it does
> - Add VM_BUG_ON(mmap_sem)



> +/*
> + * Returns 1 if a given pmd maps a stable (not under splitting) thp,
> + * -1 if the pmd maps thp under splitting, 0 if the pmd does not map thp.
> + *
> + * Note that if it returns 1, this routine returns without unlocking page
> + * table locks. So callers must unlock them.
> + */


Seems nice clean up but... why you need to return (-1, 0, 1) ?

It seems the caller can't see the difference between -1 and 0.

Why not just return 0 (not locked) or 1 (thp found and locked) ?

Thanks,
-Kame

> +int __pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma)
> +{
> + spin_lock(&vma->vm_mm->page_table_lock);
> if (likely(pmd_trans_huge(*pmd))) {
> if (unlikely(pmd_trans_splitting(*pmd))) {
> - spin_unlock(&mm->page_table_lock);
> + spin_unlock(&vma->vm_mm->page_table_lock);
> wait_split_huge_page(vma->anon_vma, pmd);
> + return -1;
> } else {
> - pmd_t entry;
> -
> - entry = pmdp_get_and_clear(mm, addr, pmd);
> - entry = pmd_modify(entry, newprot);
> - set_pmd_at(mm, addr, pmd, entry);
> - spin_unlock(&vma->vm_mm->page_table_lock);
> - ret = 1;
> + /* Thp mapped by 'pmd' is stable, so we can
> + * handle it as it is. */
> + return 1;
> }
> - } else
> - spin_unlock(&vma->vm_mm->page_table_lock);
> -
> - return ret;
> + }
> + spin_unlock(&vma->vm_mm->page_table_lock);
> + return 0;
> }
>
> pmd_t *page_check_address_pmd(struct page *page,
> --
> 1.7.7.6
>
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