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Date: Wed, 9 Jun 2010 14:22:46 +0100
From: Mel Gorman <mel@....ul.ie>
To: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Andrea Arcangeli <aarcange@...hat.com>,
Avi Kivity <avi@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
akpm@...ux-foundation.org,
Linus Torvalds <torvalds@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
David Miller <davem@...emloft.net>,
Hugh Dickins <hugh.dickins@...cali.co.uk>,
Nick Piggin <npiggin@...e.de>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 10/28] mm: Make use of the anon_vma ref count
On Mon, Jun 07, 2010 at 01:07:04PM +0200, Peter Zijlstra wrote:
> This patch changes the anon_vma refcount to be 0 when the object is
> free. It does this by adding 1 ref to being in use in the anon_vma
> structure (iow. the anon_vma->head list is not empty).
>
> This allows a simpler release scheme without having to check both the
> refcount and the list as well as avoids taking a ref for each entry
> on the list.
>
Nice touch. It makes it closer to page reference counting as well so
should be familiar.
> We then convert page_lock_anon_vma() over to use refcounts. This is
> done for each of convertion of anon_vma from spinlock to mutex.
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
> include/linux/rmap.h | 11 ++++-
> mm/ksm.c | 4 --
> mm/rmap.c | 94 ++++++++++++++++++++++-----------------------------
> 3 files changed, 52 insertions(+), 57 deletions(-)
>
> Index: linux-2.6/include/linux/rmap.h
> ===================================================================
> --- linux-2.6.orig/include/linux/rmap.h
> +++ linux-2.6/include/linux/rmap.h
> @@ -73,7 +73,13 @@ static inline void get_anon_vma(struct a
> atomic_inc(&anon_vma->refcount);
> }
>
> -void put_anon_vma(struct anon_vma *);
> +void __put_anon_vma(struct anon_vma *anon_vma);
> +
> +static inline void put_anon_vma(struct anon_vma *anon_vma)
> +{
> + if (atomic_dec_and_test(&anon_vma->refcount))
> + __put_anon_vma(anon_vma);
> +}
>
> static inline struct anon_vma *page_anon_vma(struct page *page)
> {
> @@ -116,7 +122,6 @@ void unlink_anon_vmas(struct vm_area_str
> int anon_vma_clone(struct vm_area_struct *, struct vm_area_struct *);
> int anon_vma_fork(struct vm_area_struct *, struct vm_area_struct *);
> void __anon_vma_link(struct vm_area_struct *);
> -void anon_vma_free(struct anon_vma *);
>
> static inline void anon_vma_merge(struct vm_area_struct *vma,
> struct vm_area_struct *next)
> @@ -125,6 +130,8 @@ static inline void anon_vma_merge(struct
> unlink_anon_vmas(next);
> }
>
> +struct anon_vma *page_get_anon_vma(struct page *page);
> +
> /*
> * rmap interfaces called when adding or removing pte of page
> */
> Index: linux-2.6/mm/ksm.c
> ===================================================================
> --- linux-2.6.orig/mm/ksm.c
> +++ linux-2.6/mm/ksm.c
> @@ -323,9 +323,7 @@ static void hold_anon_vma(struct rmap_it
>
> static void ksm_put_anon_vma(struct rmap_item *rmap_item)
> {
> - struct anon_vma *anon_vma = rmap_item->anon_vma;
> -
> - put_anon_vma(anon_vma);
> + put_anon_vma(rmap_item->anon_vma);
> }
>
> /*
> Index: linux-2.6/mm/rmap.c
> ===================================================================
> --- linux-2.6.orig/mm/rmap.c
> +++ linux-2.6/mm/rmap.c
> @@ -66,11 +66,24 @@ static struct kmem_cache *anon_vma_chain
>
> static inline struct anon_vma *anon_vma_alloc(void)
> {
> - return kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
> + struct anon_vma *anon_vma;
> +
> + anon_vma = kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
> + if (anon_vma) {
> + atomic_set(&anon_vma->refcount, 1);
> + /*
> + * This VMA had no anon_vma yet. This anon_vma is
> + * the root of any anon_vma tree that might form.
> + */
> + anon_vma->root = anon_vma;
> + }
This comment is no longer strictly correct. anon_vma_alloc() can also be
called from anon_vma_fork where the root of the newly allocated anon_vma
will be set to the same as the parents anon_vma. Maybe something like
/*
* Initialise the anon_vma root to point to itself. If called from
* fork, the root will be reset to the parents anon_vma
*/
Else, just leave the initialisation of the root where it was but only
init the refcount here.
> +
> + return anon_vma;
> }
>
> -void anon_vma_free(struct anon_vma *anon_vma)
> +static inline void anon_vma_free(struct anon_vma *anon_vma)
> {
> + VM_BUG_ON(atomic_read(&anon_vma->refcount));
> kmem_cache_free(anon_vma_cachep, anon_vma);
> }
>
> @@ -132,11 +145,6 @@ int anon_vma_prepare(struct vm_area_stru
> if (unlikely(!anon_vma))
> goto out_enomem_free_avc;
> allocated = anon_vma;
> - /*
> - * This VMA had no anon_vma yet. This anon_vma is
> - * the root of any anon_vma tree that might form.
> - */
> - anon_vma->root = anon_vma;
> }
>
> anon_vma_lock(anon_vma);
> @@ -155,7 +163,7 @@ int anon_vma_prepare(struct vm_area_stru
> anon_vma_unlock(anon_vma);
>
> if (unlikely(allocated))
> - anon_vma_free(allocated);
> + put_anon_vma(allocated);
> if (unlikely(avc))
> anon_vma_chain_free(avc);
> }
> @@ -248,7 +256,7 @@ int anon_vma_fork(struct vm_area_struct
> return 0;
>
> out_error_free_anon_vma:
> - anon_vma_free(anon_vma);
> + put_anon_vma(anon_vma);
> out_error:
> unlink_anon_vmas(vma);
> return -ENOMEM;
> @@ -265,16 +273,12 @@ static void anon_vma_unlink(struct anon_
>
> anon_vma_lock(anon_vma);
> list_del(&anon_vma_chain->same_anon_vma);
> -
> /* We must garbage collect the anon_vma if it's empty */
Unnecessary whitespace change there.
> - empty = list_empty(&anon_vma->head) && !atomic_read(&anon_vma->refcount);
> + empty = list_empty(&anon_vma->head);
> anon_vma_unlock(anon_vma);
>
> - if (empty) {
> - /* We no longer need the root anon_vma */
> - put_anon_vma(anon_vma->root);
> - anon_vma_free(anon_vma);
> - }
> + if (empty)
> + put_anon_vma(anon_vma);
> }
>
> void unlink_anon_vmas(struct vm_area_struct *vma)
> @@ -326,9 +330,9 @@ void __init anon_vma_init(void)
> * that the anon_vma pointer from page->mapping is valid if there is a
> * mapcount, we can dereference the anon_vma after observing those.
> */
> -struct anon_vma *page_lock_anon_vma(struct page *page)
> +struct anon_vma *page_get_anon_vma(struct page *page)
> {
> - struct anon_vma *anon_vma;
> + struct anon_vma *anon_vma = NULL;
> unsigned long anon_mapping;
>
It's not clear why you split page_get_anon_vma and page_lock_anon_vma at
this point. Maybe you use it later in the series?
> rcu_read_lock();
> @@ -339,17 +343,28 @@ struct anon_vma *page_lock_anon_vma(stru
> goto out;
>
> anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
> - anon_vma_lock(anon_vma);
> - return anon_vma;
> + if (!atomic_inc_not_zero(&anon_vma->refcount))
> + anon_vma = NULL;
> out:
> rcu_read_unlock();
> - return NULL;
> +
> + return anon_vma;
> +}
> +
> +struct anon_vma *page_lock_anon_vma(struct page *page)
> +{
> + struct anon_vma *anon_vma = page_get_anon_vma(page);
> +
> + if (anon_vma)
> + anon_vma_lock(anon_vma);
> +
> + return anon_vma;
> }
>
> void page_unlock_anon_vma(struct anon_vma *anon_vma)
> {
> anon_vma_unlock(anon_vma);
> - rcu_read_unlock();
> + put_anon_vma(anon_vma);
> }
>
> /*
> @@ -1405,36 +1420,11 @@ int try_to_munlock(struct page *page)
> return try_to_unmap_file(page, TTU_MUNLOCK);
> }
>
> -/*
> - * Drop an anon_vma refcount, freeing the anon_vma and anon_vma->root
> - * if necessary. Be careful to do all the tests under the lock. Once
> - * we know we are the last user, nobody else can get a reference and we
> - * can do the freeing without the lock.
> - */
> -void put_anon_vma(struct anon_vma *anon_vma)
> -{
> - if (atomic_dec_and_lock(&anon_vma->refcount, &anon_vma->root->lock)) {
> - struct anon_vma *root = anon_vma->root;
> - int empty = list_empty(&anon_vma->head);
> - int last_root_user = 0;
> - int root_empty = 0;
> -
> - /*
> - * The refcount on a non-root anon_vma got dropped. Drop
> - * the refcount on the root and check if we need to free it.
> - */
> - if (empty && anon_vma != root) {
> - last_root_user = atomic_dec_and_test(&root->refcount);
> - root_empty = list_empty(&root->head);
> - }
> - anon_vma_unlock(anon_vma);
> -
> - if (empty) {
> - anon_vma_free(anon_vma);
> - if (root_empty && last_root_user)
> - anon_vma_free(root);
> - }
> - }
> +void __put_anon_vma(struct anon_vma *anon_vma)
> +{
> + if (anon_vma->root != anon_vma)
> + put_anon_vma(anon_vma->root);
> + anon_vma_free(anon_vma);
> }
>
> #ifdef CONFIG_MIGRATION
>
In the first reading, nothing horrible jumped out at this point. I'm going
to run a few tests on just the first 10 patches in this series and see what
that looks like. For this patch in particular, I want to be sure we are
not leaking anon_vma's.
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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