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Message-Id: <AF34FDF1-CAD9-43FD-B97D-F6A7439F310A@joelfernandes.org>
Date: Fri, 1 Sep 2023 12:41:24 -0400
From: Joel Fernandes <joel@...lfernandes.org>
To: Uladzislau Rezki <urezki@...il.com>
Cc: linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Christoph Hellwig <hch@...radead.org>,
Lorenzo Stoakes <lstoakes@...il.com>,
Zhen Lei <thunder.leizhen@...weicloud.com>,
"Paul E . McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
Zqiang <qiang.zhang1211@...il.com>,
Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org
Subject: Re: [PATCH v2 1/2] mm/vmalloc: Add a safer version of find_vm_area() for debug
> On Sep 1, 2023, at 8:48 AM, Uladzislau Rezki <urezki@...il.com> wrote:
>
> On Fri, Sep 01, 2023 at 12:33:21AM +0000, Joel Fernandes wrote:
>>> On Fri, Sep 01, 2023 at 12:19:17AM +0000, Joel Fernandes wrote:
>>> On Thu, Aug 31, 2023 at 09:47:52PM +0200, Uladzislau Rezki wrote:
>>>> On Thu, Aug 31, 2023 at 05:18:25PM +0000, Joel Fernandes (Google) wrote:
>>>>> It is unsafe to dump vmalloc area information when trying to do so from
>>>>> some contexts. Add a safer trylock version of the same function to do a
>>>>> best-effort VMA finding and use it from vmalloc_dump_obj().
>>>>>
>>>>> [apply test robot feedback on unused function fix.]
>>>>>
>>>>> Reported-by: Zhen Lei <thunder.leizhen@...weicloud.com>
>>>>> Cc: Paul E. McKenney <paulmck@...nel.org>
>>>>> Cc: rcu@...r.kernel.org
>>>>> Cc: Zqiang <qiang.zhang1211@...il.com>
>>>>> Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>
>>>>> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>>>>> ---
>>>>> v1->v2: Apply review tags and test robot feedback.
>>>>>
>>>>> mm/vmalloc.c | 39 ++++++++++++++++++++++++++++++++++++++-
>>>>> 1 file changed, 38 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>>>>> index 93cf99aba335..f09e882ae3b8 100644
>>>>> --- a/mm/vmalloc.c
>>>>> +++ b/mm/vmalloc.c
>>>>> @@ -1865,6 +1865,20 @@ struct vmap_area *find_vmap_area(unsigned long addr)
>>>>> return va;
>>>>> }
>>>>>
>>>>> +#ifdef CONFIG_PRINTK
>>>>> +static struct vmap_area *find_vmap_area_trylock(unsigned long addr)
>>>>> +{
>>>>> + struct vmap_area *va;
>>>>> +
>>>>> + if (!spin_trylock(&vmap_area_lock))
>>>>> + return NULL;
>>>>> + va = __find_vmap_area(addr, &vmap_area_root);
>>>>> + spin_unlock(&vmap_area_lock);
>>>>> +
>>>>> + return va;
>>>>> +}
>>>>> +#endif
>>>>> +
>>>>> static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
>>>>> {
>>>>> struct vmap_area *va;
>>>>> @@ -2671,6 +2685,29 @@ struct vm_struct *find_vm_area(const void *addr)
>>>>> return va->vm;
>>>>> }
>>>>>
>>>>> +/**
>>>>> + * try_to_find_vm_area - find a continuous kernel virtual area
>>>>> + * @addr: base address
>>>>> + *
>>>>> + * This function is the same as find_vm_area() except that it is
>>>>> + * safe to call if vmap_area_lock is already held and returns NULL
>>>>> + * if it is. See comments in find_vmap_area() for other details.
>>>>> + *
>>>>> + * Return: the area descriptor on success or %NULL on failure.
>>>>> + */
>>>>> +#ifdef CONFIG_PRINTK
>>>>> +static struct vm_struct *try_to_find_vm_area(const void *addr)
>>>>> +{
>>>>> + struct vmap_area *va;
>>>>> +
>>>>> + va = find_vmap_area_trylock((unsigned long)addr);
>>>>> + if (!va)
>>>>> + return NULL;
>>>>> +
>>>>> + return va->vm;
>>>>> +}
>>>>> +#endif
>>>>> +
>>>>> /**
>>>>> * remove_vm_area - find and remove a continuous kernel virtual area
>>>>> * @addr: base address
>>>>> @@ -4277,7 +4314,7 @@ bool vmalloc_dump_obj(void *object)
>>>>> struct vm_struct *vm;
>>>>> void *objp = (void *)PAGE_ALIGN((unsigned long)object);
>>>>>
>>>>> - vm = find_vm_area(objp);
>>>>> + vm = try_to_find_vm_area(objp);
>>>>> if (!vm)
>>>>> return false;
>>>>> pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",
>>>
>>> Hi Vlad,
>>> Thanks for taking a look.
>>>
>>>> I am not sure if this patch makes a lot of sense. I agree, this is a
>>>> problem and it mitigates it. But it is broken in terms of once you drop
>>>> the lock, the VA should not be accessed.
>>>
>>> Just to note the lockless-access issue you are referring to is not introduced
>>> by this patch but is rather in the existing code. Also just to note this is
>>> debug code.
>>>
>>>> Is that a real issue or it gets triggered due to some syntetic test case?
>>>
>>> It is a real issue. See 2/2.
>>>
>>>> If i were you, i would go with open-coded version of trylock. Because
>>>> there is only one user so far.
>>>
>>> Taking your open coding and locking suggestions, I came up with the below
>>> which actually results in a smaller patch. Does it look good to you?
>>>
>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>>> index 93cf99aba335..aaf6bad997a7 100644
>>
>> And with some trivial compiler errors fixed (sorry should have build tested
>> but wanted to just share the idea earlier):
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index 93cf99aba335..2c6a0e2ff404 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -4274,14 +4274,32 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
>> #ifdef CONFIG_PRINTK
>> bool vmalloc_dump_obj(void *object)
>> {
>> - struct vm_struct *vm;
>> void *objp = (void *)PAGE_ALIGN((unsigned long)object);
>> + const void *caller;
>> + struct vm_struct *vm;
>> + struct vmap_area *va;
>> + unsigned long addr;
>> + unsigned int nr_pages;
>>
>> - vm = find_vm_area(objp);
>> - if (!vm)
>> + if (!spin_trylock(&vmap_area_lock))
>> + return false;
>> + va = __find_vmap_area((unsigned long)objp, &vmap_area_root);
>> + if (!va) {
>> + spin_unlock(&vmap_area_lock);
>> return false;
>> + }
>> +
>> + vm = va->vm;
>> + if (!vm) {
>> + spin_unlock(&vmap_area_lock);
>> + return false;
>> + }
>> + addr = (unsigned long)vm->addr;
>> + caller = vm->caller;
>> + nr_pages = vm->nr_pages;
>> + spin_unlock(&vmap_area_lock);
>> pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",
>> - vm->nr_pages, (unsigned long)vm->addr, vm->caller);
>> + nr_pages, addr, caller);
>> return true;
>> }
>> #endif
>>
> Looks good to me and thank you for fixing a locking issue :)
> I think you will re-spin and resend it one more time?
Yes. May I add your Reviewed-by tag to both patches after re-spinning as mentioned above?
thanks!
- Joel
>
> --
> Uladzislau Rezki
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