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Date: Tue, 16 Dec 2014 11:42:18 +0100
From: Michal Hocko <mhocko@...e.cz>
To: Andrew Morton <akpm@...ux-foundation.org>,
Konstantin Khlebnikov <koct9i@...il.com>
Cc: linux-mm <linux-mm@...ck.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Rik van Riel <riel@...hat.com>,
Tim Hartrick <tim@...ecast.com>,
Daniel Forrest <dan.forrest@...c.wisc.edu>,
Hugh Dickins <hughd@...gle.com>,
Michel Lespinasse <walken@...gle.com>,
Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v3] mm: prevent endless growth of anon_vma hierarchy
What happened to this patch? I do not see it merged for 3.19 and
nor in the current mmotm tree (2014-12-15-17-05)
On Wed 26-11-14 22:11:45, Konstantin Khlebnikov wrote:
> Constantly forking task causes unlimited grow of anon_vma chain.
> Each next child allocate new level of anon_vmas and links vmas to all
> previous levels because it inherits pages from them. None of anon_vmas
> cannot be freed because there might be pages which points to them.
>
> This patch adds heuristic which decides to reuse existing anon_vma instead
> of forking new one. It counts vmas and direct descendants for each anon_vma.
> Anon_vma with degree lower than two will be reused at next fork.
>
> As a result each anon_vma has either alive vma or at least two descendants,
> endless chains are no longer possible and count of anon_vmas is no more than
> two times more than count of vmas.
>
> Signed-off-by: Konstantin Khlebnikov <koct9i@...il.com>
> Reported-by: Daniel Forrest <dan.forrest@...c.wisc.edu>
> Tested-by: Michal Hocko <mhocko@...e.cz>
> Reviewed-by: Michal Hocko <mhocko@...e.cz>
> Link: http://lkml.kernel.org/r/20120816024610.GA5350@evergreen.ssec.wisc.edu
> Fixes: 5beb49305251 ("mm: change anon_vma linking to fix multi-process server scalability issue")
> Cc: Stable <stable@...r.kernel.org> (2.6.34+)
>
> ---
>
> v2: update degree in anon_vma_prepare for merged anon_vma
> v3: update comment and tags
> ---
> include/linux/rmap.h | 16 ++++++++++++++++
> mm/rmap.c | 30 +++++++++++++++++++++++++++++-
> 2 files changed, 45 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index c0c2bce..b1d140c 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -45,6 +45,22 @@ struct anon_vma {
> * mm_take_all_locks() (mm_all_locks_mutex).
> */
> struct rb_root rb_root; /* Interval tree of private "related" vmas */
> +
> + /*
> + * Count of child anon_vmas and VMAs which points to this anon_vma.
> + *
> + * This counter is used for making decision about reusing old anon_vma
> + * instead of forking new one. It allows to detect anon_vmas which have
> + * just one direct descendant and no vmas. Reusing such anon_vma not
> + * leads to significant preformance regression but prevents degradation
> + * of anon_vma hierarchy to endless linear chain.
> + *
> + * Root anon_vma is never reused because it is its own parent and it has
> + * at leat one vma or child, thus at fork it's degree is at least 2.
> + */
> + unsigned degree;
> +
> + struct anon_vma *parent; /* Parent of this anon_vma */
> };
>
> /*
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 19886fb..df5c44e 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -72,6 +72,8 @@ static inline struct anon_vma *anon_vma_alloc(void)
> anon_vma = kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
> if (anon_vma) {
> atomic_set(&anon_vma->refcount, 1);
> + anon_vma->degree = 1; /* Reference for first vma */
> + anon_vma->parent = anon_vma;
> /*
> * Initialise the anon_vma root to point to itself. If called
> * from fork, the root will be reset to the parents anon_vma.
> @@ -188,6 +190,8 @@ int anon_vma_prepare(struct vm_area_struct *vma)
> if (likely(!vma->anon_vma)) {
> vma->anon_vma = anon_vma;
> anon_vma_chain_link(vma, avc, anon_vma);
> + /* vma link if merged or child link for new root */
> + anon_vma->degree++;
> allocated = NULL;
> avc = NULL;
> }
> @@ -256,7 +260,17 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
> anon_vma = pavc->anon_vma;
> root = lock_anon_vma_root(root, anon_vma);
> anon_vma_chain_link(dst, avc, anon_vma);
> +
> + /*
> + * Reuse existing anon_vma if its degree lower than two,
> + * that means it has no vma and just one anon_vma child.
> + */
> + if (!dst->anon_vma && anon_vma != src->anon_vma &&
> + anon_vma->degree < 2)
> + dst->anon_vma = anon_vma;
> }
> + if (dst->anon_vma)
> + dst->anon_vma->degree++;
> unlock_anon_vma_root(root);
> return 0;
>
> @@ -279,6 +293,9 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
> if (!pvma->anon_vma)
> return 0;
>
> + /* Drop inherited anon_vma, we'll reuse old one or allocate new. */
> + vma->anon_vma = NULL;
> +
> /*
> * First, attach the new VMA to the parent VMA's anon_vmas,
> * so rmap can find non-COWed pages in child processes.
> @@ -286,6 +303,10 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
> if (anon_vma_clone(vma, pvma))
> return -ENOMEM;
>
> + /* An old anon_vma has been reused. */
> + if (vma->anon_vma)
> + return 0;
> +
> /* Then add our own anon_vma. */
> anon_vma = anon_vma_alloc();
> if (!anon_vma)
> @@ -299,6 +320,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
> * lock any of the anon_vmas in this anon_vma tree.
> */
> anon_vma->root = pvma->anon_vma->root;
> + anon_vma->parent = pvma->anon_vma;
> /*
> * With refcounts, an anon_vma can stay around longer than the
> * process it belongs to. The root anon_vma needs to be pinned until
> @@ -309,6 +331,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
> vma->anon_vma = anon_vma;
> anon_vma_lock_write(anon_vma);
> anon_vma_chain_link(vma, avc, anon_vma);
> + anon_vma->parent->degree++;
> anon_vma_unlock_write(anon_vma);
>
> return 0;
> @@ -339,12 +362,16 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
> * Leave empty anon_vmas on the list - we'll need
> * to free them outside the lock.
> */
> - if (RB_EMPTY_ROOT(&anon_vma->rb_root))
> + if (RB_EMPTY_ROOT(&anon_vma->rb_root)) {
> + anon_vma->parent->degree--;
> continue;
> + }
>
> list_del(&avc->same_vma);
> anon_vma_chain_free(avc);
> }
> + if (vma->anon_vma)
> + vma->anon_vma->degree--;
> unlock_anon_vma_root(root);
>
> /*
> @@ -355,6 +382,7 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
> list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
> struct anon_vma *anon_vma = avc->anon_vma;
>
> + BUG_ON(anon_vma->degree);
> put_anon_vma(anon_vma);
>
> list_del(&avc->same_vma);
>
> --
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--
Michal Hocko
SUSE Labs
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