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

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 19 Jul 2011 15:24:23 +1000

On Tue, 2011-07-19 at 13:17 +0800, Shan Hai wrote:

> The patch works, but I have certain confusions,
> - Do we want to handle_mm_fault on each futex_lock_pi
>      even though in most cases there is no write permission
>      fixup's needed?

Don't we only ever call this when futex_atomic_op_inuser() failed ?
Which means a fixup -is- needed .... The fast path is still there.

> - How about let the archs do their own write permission
>      fixup as what I did in my original

Why ? This is generic and will fix all archs at once with generic code
which is a significant improvement in my book and a lot more
maintainable :-)

>      "[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.

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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