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 12:55:04 +0800

On 07/19/2011 12:29 PM, Benjamin Herrenschmidt wrote:
> The futex code currently attempts to write to user memory within
> a pagefault disabled section, and if that fails, tries to fix it
> up using get_user_pages().
>
> This doesn't work on archs where the dirty and young bits are
> maintained by software, since they will gate access permission
> in the TLB, and will not be updated by gup().
>
> In addition, there's an expectation on some archs that a
> spurious write fault triggers a local TLB flush, and that is
> missing from the picture as well.
>
> I decided that adding those "features" to gup() would be too much
> for this already too complex function, and instead added a new
> simpler fixup_user_fault() which is essentially a wrapper around
> handle_mm_fault() which the futex code can call.
>
> Signed-off-by: Benjamin Herrenschmidt<[email-masked]>
> ---
>
> Shan, can you test this ? It might not fix the problem since I'm
> starting to have the nasty feeling that you are hitting what is
> somewhat a subtly different issue or my previous patch should
> have worked (but then I might have done a stupid mistake as well)
> but let us know anyway.
>

Ok, I will test the patch, I think this should work, because
it's similar to my first posted patch, the difference is that
I tried to do it in the futex_atomic_cmpxchg_inatomic() in
the ppc specific path, lower level than yours as in
fault_in_user_writable :-)

Anyway, I will notify you on the test result.

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.
>
>