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Date: Mon, 30 Nov 2009 09:04:48 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Izik Eidus <ieidus@...hat.com>,
Andrea Arcangeli <aarcange@...hat.com>,
Chris Wright <chrisw@...hat.com>,
Balbir Singh <balbir@...ux.vnet.ibm.com>,
Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 5/9] ksm: share anon page without allocating
Sorry for delayed response.
On Tue, 24 Nov 2009 16:48:46 +0000 (GMT)
Hugh Dickins <hugh.dickins@...cali.co.uk> wrote:
> When ksm pages were unswappable, it made no sense to include them in
> mem cgroup accounting; but now that they are swappable (although I see
> no strict logical connection)
I asked that for throwing away too complicated but wast of time things.
If not on LRU, its own limitation (ksm's page limit) works enough.
> the principle of least surprise implies
> that they should be accounted (with the usual dissatisfaction, that a
> shared page is accounted to only one of the cgroups using it).
>
> This patch was intended to add mem cgroup accounting where necessary;
> but turned inside out, it now avoids allocating a ksm page, instead
> upgrading an anon page to ksm - which brings its existing mem cgroup
> accounting with it. Thus mem cgroups don't appear in the patch at all.
>
ok. then, what I should see is patch 6.
Thanks,
-Kame
> This upgrade from PageAnon to PageKsm takes place under page lock
> (via a somewhat hacky NULL kpage interface), and audit showed only
> one place which needed to cope with the race - page_referenced() is
> sometimes used without page lock, so page_lock_anon_vma() needs an
> ACCESS_ONCE() to be sure of getting anon_vma and flags together
> (no problem if the page goes ksm an instant after, the integrity
> of that anon_vma list is unaffected).
>
> Signed-off-by: Hugh Dickins <hugh.dickins@...cali.co.uk>
> ---
>
> mm/ksm.c | 67 ++++++++++++++++------------------------------------
> mm/rmap.c | 6 +++-
> 2 files changed, 25 insertions(+), 48 deletions(-)
>
> --- ksm4/mm/ksm.c 2009-11-22 20:40:18.000000000 +0000
> +++ ksm5/mm/ksm.c 2009-11-22 20:40:27.000000000 +0000
> @@ -831,7 +831,8 @@ out:
> * try_to_merge_one_page - take two pages and merge them into one
> * @vma: the vma that holds the pte pointing to page
> * @page: the PageAnon page that we want to replace with kpage
> - * @kpage: the PageKsm page that we want to map instead of page
> + * @kpage: the PageKsm page that we want to map instead of page,
> + * or NULL the first time when we want to use page as kpage.
> *
> * This function returns 0 if the pages were merged, -EFAULT otherwise.
> */
> @@ -864,15 +865,24 @@ static int try_to_merge_one_page(struct
> * ptes are necessarily already write-protected. But in either
> * case, we need to lock and check page_count is not raised.
> */
> - if (write_protect_page(vma, page, &orig_pte) == 0 &&
> - pages_identical(page, kpage))
> - err = replace_page(vma, page, kpage, orig_pte);
> + if (write_protect_page(vma, page, &orig_pte) == 0) {
> + if (!kpage) {
> + /*
> + * While we hold page lock, upgrade page from
> + * PageAnon+anon_vma to PageKsm+NULL stable_node:
> + * stable_tree_insert() will update stable_node.
> + */
> + set_page_stable_node(page, NULL);
> + mark_page_accessed(page);
> + err = 0;
> + } else if (pages_identical(page, kpage))
> + err = replace_page(vma, page, kpage, orig_pte);
> + }
>
> - if ((vma->vm_flags & VM_LOCKED) && !err) {
> + if ((vma->vm_flags & VM_LOCKED) && kpage && !err) {
> munlock_vma_page(page);
> if (!PageMlocked(kpage)) {
> unlock_page(page);
> - lru_add_drain();
Is this related to memcg ?
> lock_page(kpage);
> mlock_vma_page(kpage);
> page = kpage; /* for final unlock */
> @@ -922,7 +932,7 @@ out:
> * This function returns the kpage if we successfully merged two identical
> * pages into one ksm page, NULL otherwise.
> *
> - * Note that this function allocates a new kernel page: if one of the pages
> + * Note that this function upgrades page to ksm page: if one of the pages
> * is already a ksm page, try_to_merge_with_ksm_page should be used.
> */
> static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
> @@ -930,10 +940,7 @@ static struct page *try_to_merge_two_pag
> struct rmap_item *tree_rmap_item,
> struct page *tree_page)
> {
> - struct mm_struct *mm = rmap_item->mm;
> - struct vm_area_struct *vma;
> - struct page *kpage;
> - int err = -EFAULT;
> + int err;
>
> /*
> * The number of nodes in the stable tree
> @@ -943,37 +950,10 @@ static struct page *try_to_merge_two_pag
> ksm_max_kernel_pages <= ksm_pages_shared)
> return NULL;
>
> - kpage = alloc_page(GFP_HIGHUSER);
> - if (!kpage)
> - return NULL;
> -
> - down_read(&mm->mmap_sem);
> - if (ksm_test_exit(mm))
> - goto up;
> - vma = find_vma(mm, rmap_item->address);
> - if (!vma || vma->vm_start > rmap_item->address)
> - goto up;
> -
> - copy_user_highpage(kpage, page, rmap_item->address, vma);
> -
> - SetPageDirty(kpage);
> - __SetPageUptodate(kpage);
> - SetPageSwapBacked(kpage);
> - set_page_stable_node(kpage, NULL); /* mark it PageKsm */
> - lru_cache_add_lru(kpage, LRU_ACTIVE_ANON);
> -
> - err = try_to_merge_one_page(vma, page, kpage);
> - if (err)
> - goto up;
> -
> - /* Must get reference to anon_vma while still holding mmap_sem */
> - hold_anon_vma(rmap_item, vma->anon_vma);
> -up:
> - up_read(&mm->mmap_sem);
> -
> + err = try_to_merge_with_ksm_page(rmap_item, page, NULL);
> if (!err) {
> err = try_to_merge_with_ksm_page(tree_rmap_item,
> - tree_page, kpage);
> + tree_page, page);
> /*
> * If that fails, we have a ksm page with only one pte
> * pointing to it: so break it.
> @@ -981,11 +961,7 @@ up:
> if (err)
> break_cow(rmap_item);
> }
> - if (err) {
> - put_page(kpage);
> - kpage = NULL;
> - }
> - return kpage;
> + return err ? NULL : page;
> }
>
> /*
> @@ -1244,7 +1220,6 @@ static void cmp_and_merge_page(struct pa
> stable_tree_append(rmap_item, stable_node);
> }
> unlock_page(kpage);
> - put_page(kpage);
>
> /*
> * If we fail to insert the page into the stable tree,
> --- ksm4/mm/rmap.c 2009-11-22 20:40:11.000000000 +0000
> +++ ksm5/mm/rmap.c 2009-11-22 20:40:27.000000000 +0000
> @@ -204,7 +204,7 @@ struct anon_vma *page_lock_anon_vma(stru
> unsigned long anon_mapping;
>
> rcu_read_lock();
> - anon_mapping = (unsigned long) page->mapping;
> + anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
> if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
> goto out;
> if (!page_mapped(page))
> @@ -666,7 +666,9 @@ static void __page_check_anon_rmap(struc
> * @address: the user virtual address mapped
> *
> * The caller needs to hold the pte lock, and the page must be locked in
> - * the anon_vma case: to serialize mapping,index checking after setting.
> + * the anon_vma case: to serialize mapping,index checking after setting,
> + * and to ensure that PageAnon is not being upgraded racily to PageKsm
> + * (but PageKsm is never downgraded to PageAnon).
> */
> void page_add_anon_rmap(struct page *page,
> struct vm_area_struct *vma, unsigned long address)
--
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