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Date: Thu, 16 Jul 2009 21:00:28 +0900
From: Minchan Kim <minchan.kim@...il.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: linux-mm@...ck.org,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
npiggin@...e.de,
"hugh.dickins@...cali.co.uk" <hugh.dickins@...cali.co.uk>,
avi@...hat.com,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
torvalds@...ux-foundation.org, aarcange@...hat.com
Subject: Re: [PATCH 2/2] ZERO PAGE based on pte_special
Hi, Kame.
On Thu, 16 Jul 2009 18:04:24 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>
> ZERO_PAGE for anonymous private mapping is useful when an application
> requires large continuous memory but write sparsely or some other usages.
> It was removed in 2.6.24 but this patch tries to re-add it.
> (Because there are some use cases..)
>
> In past, ZERO_PAGE was removed because heavy cache line contention in
> ZERO_PAGE's refcounting, this version of ZERO_PAGE avoid to refcnt it.
> Then, implementation is changed as following.
>
> - Use of ZERO_PAGE is limited to PRIVATE mapping. Then, VM_MAYSHARE is
> checked as VM_SHARED.
>
> - pte_special(), _PAGE_SPECIAL bit in pte is used for indicating ZERO_PAGE.
>
> - vm_normal_page() eats FOLL_XXX flag. If FOLL_NOZERO is set,
> NULL is returned even if ZERO_PAGE is found.
>
> - __get_user_pages() eats one more flag as GUP_FLAGS_IGNORE_ZERO. If set,
> __get_user_page() returns NULL even if ZERO_PAGE is found.
>
> Note:
> - no changes to get_user_pages(). ZERO_PAGE can be returned when
> vma is ANONYMOUS && PRIVATE and the access is READ.
>
> Changelog v3->v4
> - FOLL_NOZERO is directly passed to vm_normal_page()
>
> Changelog v2->v3
> - totally renewed.
> - use pte_special()
> - added new argument to vm_normal_page().
> - MAYSHARE is checked.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
> fs/proc/task_mmu.c | 8 +--
> include/linux/mm.h | 3 -
> mm/fremap.c | 2
> mm/internal.h | 1
> mm/memory.c | 136 +++++++++++++++++++++++++++++++++++++++++------------
> mm/mempolicy.c | 8 +--
> mm/migrate.c | 6 +-
> mm/mlock.c | 2
> mm/rmap.c | 6 +-
> 9 files changed, 128 insertions(+), 44 deletions(-)
>
> Index: mmotm-2.6.31-Jul15/mm/memory.c
> ===================================================================
> --- mmotm-2.6.31-Jul15.orig/mm/memory.c
> +++ mmotm-2.6.31-Jul15/mm/memory.c
> @@ -442,6 +442,27 @@ static inline int is_cow_mapping(unsigne
> }
>
> /*
> + * Can we use ZERO_PAGE at fault ? or Can we do the FOLL_ANON optimization ?
> + */
> +static inline int use_zero_page(struct vm_area_struct *vma)
> +{
> + /*
> + * We don't want to optimize FOLL_ANON for make_pages_present()
> + * when it tries to page in a VM_LOCKED region. As to VM_SHARED,
> + * we want to get the page from the page tables to make sure
> + * that we serialize and update with any other user of that
> + * mapping. At doing page fault, VM_MAYSHARE should be also check for
> + * avoiding possible changes to VM_SHARED.
> + */
> + if (vma->vm_flags & (VM_LOCKED | VM_SHARED | VM_MAYSHARE))
> + return 0;
> + /*
> + * And if we have a fault routine, it's not an anonymous region.
> + */
> + return !vma->vm_ops || !vma->vm_ops->fault;
> +}
> +
> +/*
> * vm_normal_page -- This function gets the "struct page" associated with a pte.
> *
> * "Special" mappings do not wish to be associated with a "struct page" (either
> @@ -488,16 +509,33 @@ static inline int is_cow_mapping(unsigne
> #else
> # define HAVE_PTE_SPECIAL 0
> #endif
> +
> +#ifdef CONFIG_SUPPORT_ANON_ZERO_PAGE
> +# define HAVE_ANON_ZERO 1
> +#else
> +# define HAVE_ANON_ZERO 0
> +#endif
> struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
> - pte_t pte)
> + pte_t pte, unsigned int flags)
> {
> unsigned long pfn = pte_pfn(pte);
>
> if (HAVE_PTE_SPECIAL) {
> if (likely(!pte_special(pte)))
> goto check_pfn;
> - if (!(vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP)))
> - print_bad_pte(vma, addr, pte, NULL);
> +
> + if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
> + return NULL;
> + /*
> + * ZERO PAGE ? If vma is shared or has page fault handler,
> + * Using ZERO PAGE is bug.
> + */
> + if (HAVE_ANON_ZERO && use_zero_page(vma)) {
> + if (flags & FOLL_NOZERO)
> + return NULL;
> + return ZERO_PAGE(0);
> + }
> + print_bad_pte(vma, addr, pte, NULL);
> return NULL;
> }
>
> @@ -591,8 +629,7 @@ copy_one_pte(struct mm_struct *dst_mm, s
> if (vm_flags & VM_SHARED)
> pte = pte_mkclean(pte);
> pte = pte_mkold(pte);
> -
> - page = vm_normal_page(vma, addr, pte);
> + page = vm_normal_page(vma, addr, pte, FOLL_NOZERO);
> if (page) {
> get_page(page);
> page_dup_rmap(page, vma, addr);
> @@ -783,7 +820,7 @@ static unsigned long zap_pte_range(struc
> if (pte_present(ptent)) {
> struct page *page;
>
> - page = vm_normal_page(vma, addr, ptent);
> + page = vm_normal_page(vma, addr, ptent, FOLL_NOZERO);
> if (unlikely(details) && page) {
> /*
> * unmap_shared_mapping_pages() wants to
> @@ -1141,7 +1178,7 @@ struct page *follow_page(struct vm_area_
> goto no_page;
> if ((flags & FOLL_WRITE) && !pte_write(pte))
> goto unlock;
> - page = vm_normal_page(vma, address, pte);
> + page = vm_normal_page(vma, address, pte, flags);
> if (unlikely(!page))
> goto bad_page;
>
> @@ -1186,23 +1223,6 @@ no_page_table:
> return page;
> }
>
> -/* Can we do the FOLL_ANON optimization? */
> -static inline int use_zero_page(struct vm_area_struct *vma)
> -{
> - /*
> - * We don't want to optimize FOLL_ANON for make_pages_present()
> - * when it tries to page in a VM_LOCKED region. As to VM_SHARED,
> - * we want to get the page from the page tables to make sure
> - * that we serialize and update with any other user of that
> - * mapping.
> - */
> - if (vma->vm_flags & (VM_LOCKED | VM_SHARED))
> - return 0;
> - /*
> - * And if we have a fault routine, it's not an anonymous region.
> - */
> - return !vma->vm_ops || !vma->vm_ops->fault;
> -}
>
>
>
> @@ -1216,6 +1236,7 @@ int __get_user_pages(struct task_struct
> int force = !!(flags & GUP_FLAGS_FORCE);
> int ignore = !!(flags & GUP_FLAGS_IGNORE_VMA_PERMISSIONS);
> int ignore_sigkill = !!(flags & GUP_FLAGS_IGNORE_SIGKILL);
> + int ignore_zero = !!(flags & GUP_FLAGS_IGNORE_ZERO);
>
> if (nr_pages <= 0)
> return 0;
> @@ -1259,7 +1280,12 @@ int __get_user_pages(struct task_struct
> return i ? : -EFAULT;
> }
> if (pages) {
> - struct page *page = vm_normal_page(gate_vma, start, *pte);
> + struct page *page;
> + /*
> + * this is not anon vma...don't haddle zero page
> + * related flags.
> + */
> + page = vm_normal_page(gate_vma, start, *pte, 0);
> pages[i] = page;
> if (page)
> get_page(page);
> @@ -1287,8 +1313,13 @@ int __get_user_pages(struct task_struct
> foll_flags = FOLL_TOUCH;
> if (pages)
> foll_flags |= FOLL_GET;
> - if (!write && use_zero_page(vma))
> - foll_flags |= FOLL_ANON;
> + if (!write) {
> + if (use_zero_page(vma))
> + foll_flags |= FOLL_ANON;
> + else
> + ignore_zero = 0;
> + } else
> + ignore_zero = 0;
Hmm. nested condition is not good for redabililty.
How about this ?
if (!write && use_zero_page(vma))
foll_flags |= FOLL_ANON;
else
ignore_zero = 0;
--
Kind regards,
Minchan Kim
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