Re: [PATCH -V5 22/25] powerpc/THP: get_user_pages_fast changes

[David Gibson <dwg@au1.ibm.com>]

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On Thu, Apr 04, 2013 at 11:28:00AM +0530, Aneesh Kumar K.V wrote:
> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> 
> handle large pages for get_user_pages_fast. Also take care of large page splitting.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> ---
>  arch/powerpc/mm/gup.c |   84 +++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 82 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/mm/gup.c b/arch/powerpc/mm/gup.c
> index d7efdbf..835c1ae 100644
> --- a/arch/powerpc/mm/gup.c
> +++ b/arch/powerpc/mm/gup.c
> @@ -55,6 +55,72 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
>  	return 1;
>  }
>  
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +static inline int gup_huge_pmd(pmd_t *pmdp, unsigned long addr,
> +			       unsigned long end, int write,
> +			       struct page **pages, int *nr)
> +{
> +	int refs;
> +	pmd_t pmd;
> +	unsigned long mask;
> +	struct page *head, *page, *tail;
> +
> +	pmd = *pmdp;
> +	mask = PMD_HUGE_PRESENT | PMD_HUGE_USER;
> +	if (write)
> +		mask |= PMD_HUGE_RW;
> +
> +	if ((pmd_val(pmd) & mask) != mask)
> +		return 0;
> +
> +	/* large pages are never "special" */
> +	VM_BUG_ON(!pfn_valid(pmd_pfn(pmd)));
> +
> +	refs = 0;
> +	head = pmd_page(pmd);
> +	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
> +	tail = page;
> +	do {
> +		VM_BUG_ON(compound_head(page) != head);
> +		pages[*nr] = page;
> +		(*nr)++;
> +		page++;
> +		refs++;
> +	} while (addr += PAGE_SIZE, addr != end);
> +
> +	if (!page_cache_add_speculative(head, refs)) {
> +		*nr -= refs;
> +		return 0;
> +	}
> +
> +	if (unlikely(pmd_val(pmd) != pmd_val(*pmdp))) {
> +		*nr -= refs;
> +		while (refs--)
> +			put_page(head);
> +		return 0;
> +	}
> +	/*
> +	 * Any tail page need their mapcount reference taken before we
> +	 * return.
> +	 */
> +	while (refs--) {
> +		if (PageTail(tail))
> +			get_huge_page_tail(tail);
> +		tail++;

Is it safe to do this accounting this late?

> +	}
> +
> +	return 1;
> +}
> +#else
> +
> +static inline int gup_huge_pmd(pmd_t *pmdp, unsigned long addr,
> +			       unsigned long end, int write,
> +			       struct page **pages, int *nr)
> +{
> +	return 1;

Should be a BUG() here, since this should never be called if
!CONFIG_TRANSPARENT_HUGEPAGE.

> +}
> +#endif
> +
>  static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
>  		int write, struct page **pages, int *nr)
>  {
> @@ -66,9 +132,23 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
>  		pmd_t pmd = *pmdp;
>  
>  		next = pmd_addr_end(addr, end);
> -		if (pmd_none(pmd))
> +		/*
> +		 * The pmd_trans_splitting() check below explains why
> +		 * pmdp_splitting_flush has to flush the tlb, to stop
> +		 * this gup-fast code from running while we set the
> +		 * splitting bit in the pmd. Returning zero will take
> +		 * the slow path that will call wait_split_huge_page()
> +		 * if the pmd is still in splitting state. gup-fast
> +		 * can't because it has irq disabled and
> +		 * wait_split_huge_page() would never return as the
> +		 * tlb flush IPI wouldn't run.
> +		 */
> +		if (pmd_none(pmd) || pmd_trans_splitting(pmd))
>  			return 0;
> -		if (is_hugepd(pmdp)) {
> +		if (unlikely(pmd_large(pmd))) {
> +			if (!gup_huge_pmd(pmdp, addr, next, write, pages, nr))
> +				return 0;
> +		} else if (is_hugepd(pmdp)) {
>  			if (!gup_hugepd((hugepd_t *)pmdp, PMD_SHIFT,
>  					addr, next, write, pages, nr))
>  				return 0;

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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