Subject: Re: [RFC/PATCH] mm/futex: Fix futex writes on archs with SW tracking of dirty & young

From: Shan Hai <[email-masked]>
Date: Tue, 19 Jul 2011 16:24:32 +0800

On 07/19/2011 03:46 PM, Benjamin Herrenschmidt wrote:
> On Tue, 2011-07-19 at 13:38 +0800, Shan Hai wrote:
>
>> What you said is another path, that is futex_wake_op(),
>> but what about futex_lock_pi in which my test case failed?
>> your patch will call handle_mm_fault on every futex contention
>> in the futex_lock_pi path.
>>
>> futex_lock_pi()
>>       ret = futex_lock_pi_atomic(uaddr, hb,&q.key,&q.pi_state, current, 0);
>>           case -EFAULT:
>>                           goto uaddr_faulted;
>>
>>       ...
>> uaddr_faulted:
>>       ret = fault_in_user_writeable(uaddr);
> Euh ... and how do we get to uaddr_faulted ? You may have missed the
> return statement right before it :-)
>
>  From what I can tell we only get there as a result of -EFAULT from
> futex_lock_pi_atomic() which is exactly the case we are trying to
> cover.
>

Got it, if the fault_in_user_writeable() is designed to catch the
exact same write permission fault problem we discuss here, so
your patch fixed that very nicely, we should fixup it by directly
calling handle_mm_fault like what you did because we are for sure
to know what just happened(permission violation), its not necessary
to check what's happened by calling gup-->follow_page, and
further the follow_page failed to report the fault :-)

Thanks
Shan Hai

>   .../...
>>>>        "[PATCH 1/1] Fixup write permission of TLB on powerpc e500 core"?
>>>>        (I will fix the stupid errors in my original patch if the concept
>>>> is acceptable)
>>>>        in this way we could decrease the overhead of handle_mm_fault
>>>>        in the path which does not need write permission fixup.
>>> Which overhead ? gup does handle_mm_fault() as well if needed.
>> it does it *if needed*, and this requirement is rare in my opinion.
> And how does gup figure out of it's needed ? By walking down the page
> tables in follow_page... what does handle_mm_fault do ? walk down the
> page tables...
>
> The main (if not the only) relevant difference here, is going to be the
> spurious fault TLB invaliate for writes ... which is a nop on x86....
> and needed in all the cases we care about (and if it's not needed, then
> it's up to the arch to nop it out, we should probably do it on powerpc
> too ...  but that's un unrelated discussion).
>
> Cheers,
> Ben.
>
>> Thanks
>> Shan Hai
>>
>>> What I do is I replace what is arguably an abuse of gup() in the case
>>> where a fixup -is- needed with a dedicated function designed to perform
>>> the said fixup ... and do it properly which gup() didn't :-)
>>>
>>> Cheers,
>>> Ben.
>>>
>>>> Thanks
>>>> Shan Hai
>>>>> Cheers,
>>>>> Ben.
>>>>>
>>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>>> index 9670f71..1036614 100644
>>>>> --- a/include/linux/mm.h
>>>>> +++ b/include/linux/mm.h
>>>>> @@ -985,6 +985,8 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>>>>>     int get_user_pages_fast(unsigned long start, int nr_pages, int write,
>>>>>     			struct page **pages);
>>>>>     struct page *get_dump_page(unsigned long addr);
>>>>> +extern int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
>>>>> +			    unsigned long address, unsigned int fault_flags);
>>>>>
>>>>>     extern int try_to_release_page(struct page * page, gfp_t gfp_mask);
>>>>>     extern void do_invalidatepage(struct page *page, unsigned long offset);
>>>>> diff --git a/kernel/futex.c b/kernel/futex.c
>>>>> index fe28dc2..7a0a4ed 100644
>>>>> --- a/kernel/futex.c
>>>>> +++ b/kernel/futex.c
>>>>> @@ -355,8 +355,8 @@ static int fault_in_user_writeable(u32 __user *uaddr)
>>>>>     	int ret;
>>>>>
>>>>>     	down_read(&mm->mmap_sem);
>>>>> -	ret = get_user_pages(current, mm, (unsigned long)uaddr,
>>>>> -			     1, 1, 0, NULL, NULL);
>>>>> +	ret = fixup_user_fault(current, mm, (unsigned long)uaddr,
>>>>> +			       FAULT_FLAG_WRITE);
>>>>>     	up_read(&mm->mmap_sem);
>>>>>
>>>>>     	return ret<    0 ? ret : 0;
>>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>>> index 40b7531..b967fb0 100644
>>>>> --- a/mm/memory.c
>>>>> +++ b/mm/memory.c
>>>>> @@ -1815,7 +1815,64 @@ next_page:
>>>>>     }
>>>>>     EXPORT_SYMBOL(__get_user_pages);
>>>>>
>>>>> -/**
>>>>> +/*
>>>>> + * fixup_user_fault() - manually resolve a user page  fault
>>>>> + * @tsk:	the task_struct to use for page fault accounting, or
>>>>> + *		NULL if faults are not to be recorded.
>>>>> + * @mm:		mm_struct of target mm
>>>>> + * @address:	user address
>>>>> + * @fault_flags:flags to pass down to handle_mm_fault()
>>>>> + *
>>>>> + * This is meant to be called in the specific scenario where for
>>>>> + * locking reasons we try to access user memory in atomic context
>>>>> + * (within a pagefault_disable() section), this returns -EFAULT,
>>>>> + * and we want to resolve the user fault before trying again.
>>>>> + *
>>>>> + * Typically this is meant to be used by the futex code.
>>>>> + *
>>>>> + * The main difference with get_user_pages() is that this function
>>>>> + * will unconditionally call handle_mm_fault() which will in turn
>>>>> + * perform all the necessary SW fixup of the dirty and young bits
>>>>> + * in the PTE, while handle_mm_fault() only guarantees to update
>>>>> + * these in the struct page.
>>>>> + *
>>>>> + * This is important for some architectures where those bits also
>>>>> + * gate the access permission to the page because their are
>>>>> + * maintained in software. On such architecture, gup() will not
>>>>> + * be enough to make a subsequent access succeed.
>>>>> + *
>>>>> + * This should be called with the mm_sem held for read.
>>>>> + */
>>>>> +int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
>>>>> +		     unsigned long address, unsigned int fault_flags)
>>>>> +{
>>>>> +	struct vm_area_struct *vma;
>>>>> +	int ret;
>>>>> +
>>>>> +	vma = find_extend_vma(mm, address);
>>>>> +	if (!vma || address<    vma->vm_start)
>>>>> +		return -EFAULT;
>>>>> +	
>>>>> +	ret = handle_mm_fault(mm, vma, address, fault_flags);
>>>>> +	if (ret&    VM_FAULT_ERROR) {
>>>>> +		if (ret&    VM_FAULT_OOM)
>>>>> +			return -ENOMEM;
>>>>> +		if (ret&    (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE))
>>>>> +			return -EHWPOISON;
>>>>> +		if (ret&    VM_FAULT_SIGBUS)
>>>>> +			return -EFAULT;
>>>>> +		BUG();
>>>>> +	}
>>>>> +	if (tsk) {
>>>>> +		if (ret&    VM_FAULT_MAJOR)
>>>>> +			tsk->maj_flt++;
>>>>> +		else
>>>>> +			tsk->min_flt++;
>>>>> +	}
>>>>> +	return 0;
>>>>> +}
>>>>> +
>>>>> +/*
>>>>>      * get_user_pages() - pin user pages in memory
>>>>>      * @tsk:	the task_struct to use for page fault accounting, or
>>>>>      *		NULL if faults are not to be recorded.
>>>>>
>>>>>
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>