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Date: Fri, 05 Dec 2014 16:44:48 +0100
From: Jerome Marchand <jmarchan@...hat.com>
To: Michal Hocko <mhocko@...e.cz>,
Konstantin Khlebnikov <koct9i@...il.com>
CC: Rik van Riel <riel@...hat.com>,
Michel Lespinasse <walken@...gle.com>,
Vlastimil Babka <vbabka@...e.cz>,
Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>,
Andrea Arcangeli <aarcange@...hat.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
Tim Hartrick <tim@...ecast.com>,
Daniel Forrest <dan.forrest@...c.wisc.edu>
Subject: Re: [PATCH] Repeated fork() causes SLAB to grow without bound
On 11/26/2014 06:35 PM, Michal Hocko wrote:
> On Tue 25-11-14 16:00:06, Michal Hocko wrote:
>> On Tue 25-11-14 16:13:16, Konstantin Khlebnikov wrote:
>>> On Tue, Nov 25, 2014 at 1:59 PM, Michal Hocko <mhocko@...e.cz> wrote:
>>>> On Mon 24-11-14 11:09:40, Konstantin Khlebnikov wrote:
>>>>> On Thu, Nov 20, 2014 at 6:03 PM, Konstantin Khlebnikov <koct9i@...il.com> wrote:
>>>>>> On Thu, Nov 20, 2014 at 5:50 PM, Rik van Riel <riel@...hat.com> wrote:
>>>>>>> -----BEGIN PGP SIGNED MESSAGE-----
>>>>>>> Hash: SHA1
>>>>>>>
>>>>>>> On 11/20/2014 09:42 AM, Konstantin Khlebnikov wrote:
>>>>>>>
>>>>>>>> I'm thinking about limitation for reusing anon_vmas which might
>>>>>>>> increase performance without breaking asymptotic estimation of
>>>>>>>> count anon_vma in the worst case. For example this heuristic: allow
>>>>>>>> to reuse only anon_vma with single direct descendant. It seems
>>>>>>>> there will be arount up to two times more anon_vmas but
>>>>>>>> false-aliasing must be much lower.
>>>>>
>>>>> Done. RFC patch in attachment.
>
> Ok, finally managed to untagnle myself from vma chains and your patch
> makes sense to me, it is quite clever actually. Here is it including the
> fixup.
> ---
>> From 1d4b0b38198c69ecfeb37670cb1dda767a802c9a Mon Sep 17 00:00:00 2001
>> From: Konstantin Khlebnikov <koct9i@...il.com>
>> Date: Tue, 25 Nov 2014 10:54:44 +0100
>> Subject: [PATCH] mm: prevent endless growth of anon_vma hierarchy
>>
>> 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>
>> Link: http://lkml.kernel.org/r/20120816024610.GA5350@evergreen.ssec.wisc.edu
>
> Tested-by: Michal Hocko <mhocko@...e.cz>
> Reviewed-by: Michal Hocko <mhocko@...e.cz>
>
> and I guess
> Reported-by: Daniel Forrest <dan.forrest@...c.wisc.edu>
Tested-by: Jerome Marchand <jmarchan@...hat.com>
Minor nitpicks below.
>
> who somehow vanished from CC list (added back) would be appropriate as
> well.
>
> plus
>
> Fixes: 5beb49305251 (mm: change anon_vma linking to fix multi-process server scalability issue)
> and mark it for stable
>
> Thanks!
>
>> ---
>> include/linux/rmap.h | 16 ++++++++++++++++
>> mm/rmap.c | 29 ++++++++++++++++++++++++++++-
>> 2 files changed, 44 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>> index c0c2bce6b0b7..b1d140c20b37 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
Does it or does it not lead to significant performance issue? I can't tell.
>> + * 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.
s/leat/least/
Thanks,
Jerome
>> + */
>> + unsigned degree;
>> +
>> + struct anon_vma *parent; /* Parent of this anon_vma */
>> };
>>
>> /*
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 19886fb2f13a..40ae8184a1e1 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 +190,7 @@ 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);
>> + anon_vma->degree++;
>> allocated = NULL;
>> avc = NULL;
>> }
>> @@ -256,7 +259,17 @@ 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 +292,9 @@ 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 +302,10 @@ 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 +319,7 @@ 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 +330,7 @@ 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 +361,16 @@ 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 +381,7 @@ 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);
>> --
>> 2.1.3
>
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