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Message-ID: <eb603fae09005e4a23c9e84da316ff68.squirrel@webmail-b.css.fujitsu.com>
Date: Thu, 16 Jul 2009 22:02:28 +0900 (JST)
From: "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
To: "Minchan Kim" <minchan.kim@...il.com>
Cc: "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
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
Minchan Kim wrote:
>
> 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;
>
Ah, yes. yours seems better.
I'll post updated one, later.
Thank you
-Kame
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