lkml.org 
[lkml]   [2012]   [Jun]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 2/6] mempolicy: remove all mempolicy sharing
    On Mon, Jun 11, 2012 at 05:17:26AM -0400, kosaki.motohiro@gmail.com wrote:
    > From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    >
    > Dave Jones' system call fuzz testing tool "trinity" triggered the following
    > bug error with slab debugging enabled
    >
    > [ 7613.229315] =============================================================================
    > [ 7613.229955] BUG numa_policy (Not tainted): Poison overwritten
    > [ 7613.230560] -----------------------------------------------------------------------------
    > [ 7613.230560]
    > [ 7613.231834] INFO: 0xffff880146498250-0xffff880146498250. First byte 0x6a instead of 0x6b
    > [ 7613.232518] INFO: Allocated in mpol_new+0xa3/0x140 age=46310 cpu=6 pid=32154
    > [ 7613.233188] __slab_alloc+0x3d3/0x445
    > [ 7613.233877] kmem_cache_alloc+0x29d/0x2b0
    > [ 7613.234564] mpol_new+0xa3/0x140
    > [ 7613.235236] sys_mbind+0x142/0x620
    > [ 7613.235929] system_call_fastpath+0x16/0x1b
    > [ 7613.236640] INFO: Freed in __mpol_put+0x27/0x30 age=46268 cpu=6 pid=32154
    > [ 7613.237354] __slab_free+0x2e/0x1de
    > [ 7613.238080] kmem_cache_free+0x25a/0x260
    > [ 7613.238799] __mpol_put+0x27/0x30
    > [ 7613.239515] remove_vma+0x68/0x90
    > [ 7613.240223] exit_mmap+0x118/0x140
    > [ 7613.240939] mmput+0x73/0x110
    > [ 7613.241651] exit_mm+0x108/0x130
    > [ 7613.242367] do_exit+0x162/0xb90
    > [ 7613.243074] do_group_exit+0x4f/0xc0
    > [ 7613.243790] sys_exit_group+0x17/0x20
    > [ 7613.244507] system_call_fastpath+0x16/0x1b
    > [ 7613.245212] INFO: Slab 0xffffea0005192600 objects=27 used=27 fp=0x (null) flags=0x20000000004080
    > [ 7613.246000] INFO: Object 0xffff880146498250 @offset=592 fp=0xffff88014649b9d0
    >
    > The problem was created by a reference count imbalance. Example, In following case,
    > mbind(addr, len) try to replace mempolicies of vma1 and vma2 and then they will
    > be share the same mempolicy, and the new mempolicy has MPOL_F_SHARED flag.
    >
    > +-------------------+-------------------+
    > | vma1 | vma2(shmem) |
    > +-------------------+-------------------+
    > | |
    > addr addr+len
    >

    Your example is missing some important detail. When I was looking at this
    I thought of the same scenario because initially I thought this might be
    the problem Dave's test case was hitting. Obviously I then proceeded to
    mess up anyway so take this with a grain of salt but why is this particular
    situation not prevented by vma_merge? is_mergeable_vma() should have spotted
    that the vm_files differed and mbind_range() should not have tried
    sharing them.

    > Look at alloc_pages_vma(), it uses get_vma_policy() and mpol_cond_put() pair
    > for maintaining mempolicy refcount. The current rule is, get_vma_policy() does
    > NOT increase a refcount if the policy is not attached shmem vma and mpol_cond_put()
    > DOES decrease a refcount if mpol has MPOL_F_SHARED.
    >

    The rules about refcounting are indeed annoying. It would be a lot easier
    to understand if the reference counting was unconditional but then every
    page allocation in a large VMA would also bounce the cacheline storing
    the count which would just generate a new bug later.

    > In above case, vma1 is not shmem vma and vma->policy has MPOL_F_SHARED! then,
    > get_vma_policy() doesn't increase a refcount and mpol_cond_put() decrease a
    > refcount whenever alloc_page_vma() is called.
    >
    > The bug was introduced by commit 52cd3b0740 (mempolicy: rework mempolicy Reference
    > Counting) at 4 years ago.
    >
    > More unfortunately mempolicy has one another serious broken. Currently,
    > mempolicy rebind logic (it is called from cpuset rebinding) ignore a refcount
    > of mempolicy and override it forcibly. Thus, any mempolicy sharing may
    > cause mempolicy corruption. The bug was introduced by commit 68860ec10b
    > (cpusets: automatic numa mempolicy rebinding) at 7 years ago.
    >

    I suspect these bugs were not noticed because the shmem policies are
    typically large and very long lived without much use of mbind() but
    that's not an excuse.

    > To disable policy sharing solves user visible breakage and this patch does it.
    > Maybe, we need to rewrite MPOL_F_SHARED and mempolicy rebinding code and aim
    > to proper cow logic eventually, but I think this is good first step.
    >
    > Reported-by: Dave Jones <davej@redhat.com>,
    > Cc: Mel Gorman <mgorman@suse.de>
    > Cc: Christoph Lameter <cl@linux.com>,
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: <stable@vger.kernel.org>
    > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    > ---
    > mm/mempolicy.c | 49 ++++++++++++++++++++++++++++++++++++-------------
    > 1 files changed, 36 insertions(+), 13 deletions(-)
    >
    > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
    > index 0a60def..9505cb9 100644
    > --- a/mm/mempolicy.c
    > +++ b/mm/mempolicy.c
    > @@ -607,24 +607,38 @@ check_range(struct mm_struct *mm, unsigned long start, unsigned long end,
    > return first;
    > }
    >
    > -/* Apply policy to a single VMA */
    > -static int policy_vma(struct vm_area_struct *vma, struct mempolicy *new)
    > +/*
    > + * Apply policy to a single VMA
    > + * This must be called with the mmap_sem held for writing.
    > + */
    > +static int policy_vma(struct vm_area_struct *vma, struct mempolicy *pol)

    If we're going to change this, change the policy_vma() name as well to
    set_vma_policy. We currently have policy_vma() and vma_policy() which mean
    totally different things which is partially why I deleted it entirely the
    first time around. It's a small issue but it might make mempolicy.c 0.0001%
    easier to follow.

    > {
    > - int err = 0;
    > - struct mempolicy *old = vma->vm_policy;
    > + int err;
    > + struct mempolicy *old;
    > + struct mempolicy *new;
    >
    > pr_debug("vma %lx-%lx/%lx vm_ops %p vm_file %p set_policy %p\n",
    > vma->vm_start, vma->vm_end, vma->vm_pgoff,
    > vma->vm_ops, vma->vm_file,
    > vma->vm_ops ? vma->vm_ops->set_policy : NULL);
    >
    > - if (vma->vm_ops && vma->vm_ops->set_policy)
    > + new = mpol_dup(pol);
    > + if (IS_ERR(new))
    > + return PTR_ERR(new);
    > +
    > + if (vma->vm_ops && vma->vm_ops->set_policy) {
    > err = vma->vm_ops->set_policy(vma, new);
    > - if (!err) {
    > - mpol_get(new);
    > - vma->vm_policy = new;
    > - mpol_put(old);
    > + if (err)
    > + goto err_out;
    > }
    > +
    > + old = vma->vm_policy;
    > + vma->vm_policy = new; /* protected by mmap_sem */
    > + mpol_put(old);
    > +
    > + return 0;
    > + err_out:
    > + mpol_put(new);
    > return err;
    > }
    >
    > @@ -2147,15 +2161,24 @@ static void sp_delete(struct shared_policy *sp, struct sp_node *n)
    > static struct sp_node *sp_alloc(unsigned long start, unsigned long end,
    > struct mempolicy *pol)
    > {
    > - struct sp_node *n = kmem_cache_alloc(sn_cache, GFP_KERNEL);
    > + struct sp_node *n;
    > + struct mempolicy *newpol;
    >
    > + n = kmem_cache_alloc(sn_cache, GFP_KERNEL);
    > if (!n)
    > return NULL;
    > +
    > + newpol = mpol_dup(pol);
    > + if (IS_ERR(newpol)) {
    > + kmem_cache_free(sn_cache, n);
    > + return NULL;
    > + }
    > + newpol->flags |= MPOL_F_SHARED;
    > +
    > n->start = start;
    > n->end = end;
    > - mpol_get(pol);
    > - pol->flags |= MPOL_F_SHARED; /* for unref */
    > - n->policy = pol;
    > + n->policy = newpol;
    > +
    > return n;
    > }
    >

    --
    Mel Gorman
    SUSE Labs


    \
     
     \ /
      Last update: 2012-06-12 16:41    [from the cache]
    ©2003-2014 Jasper Spaans. hosted at Digital Ocean