Re: [patch 01/10] emm: mm_lock: Lock a process against reclaim

[Jeremy Fitzhardinge <jeremy@goop.org>]

Christoph Lameter wrote:
> Provide a way to lock an mm_struct against reclaim (try_to_unmap
> etc). This is necessary for the invalidate notifier approaches so
> that they can reliably add and remove a notifier.
>
> Signed-off-by: Andrea Arcangeli <andrea@qumranet.com>
> Signed-off-by: Christoph Lameter <clameter@sgi.com>
>
> ---
>  include/linux/mm.h |   10 ++++++++
>  mm/mmap.c          |   66 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 76 insertions(+)
>
> Index: linux-2.6/include/linux/mm.h
> ===================================================================
> --- linux-2.6.orig/include/linux/mm.h	2008-04-02 11:41:47.741678873 -0700
> +++ linux-2.6/include/linux/mm.h	2008-04-04 15:02:17.660504756 -0700
> @@ -1050,6 +1050,16 @@ extern int install_special_mapping(struc
>  				   unsigned long addr, unsigned long len,
>  				   unsigned long flags, struct page **pages);
>  
> +/*
> + * Locking and unlocking am mm against reclaim.
> + *
> + * mm_lock will take mmap_sem writably (to prevent additional vmas from being
> + * added) and then take all mapping locks of the existing vmas. With that
> + * reclaim is effectively stopped.
> + */
> +extern void mm_lock(struct mm_struct *mm);
> +extern void mm_unlock(struct mm_struct *mm);
> +
>  extern unsigned long get_unmapped_area(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
>  
>  extern unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
> Index: linux-2.6/mm/mmap.c
> ===================================================================
> --- linux-2.6.orig/mm/mmap.c	2008-04-04 14:55:03.477593980 -0700
> +++ linux-2.6/mm/mmap.c	2008-04-04 14:59:05.505395402 -0700
> @@ -2242,3 +2242,69 @@ int install_special_mapping(struct mm_st
>  
>  	return 0;
>  }
> +
> +static void mm_lock_unlock(struct mm_struct *mm, int lock)
> +{
> +	struct vm_area_struct *vma;
> +	spinlock_t *i_mmap_lock_last, *anon_vma_lock_last;
> +
> +	i_mmap_lock_last = NULL;
> +	for (;;) {
> +		spinlock_t *i_mmap_lock = (spinlock_t *) -1UL;
> +		for (vma = mm->mmap; vma; vma = vma->vm_next)
> +			if (vma->vm_file && vma->vm_file->f_mapping &&
>   
I think you can break this if() down a bit:

			if (!(vma->vm_file && vma->vm_file->f_mapping))
				continue;


> +			    (unsigned long) i_mmap_lock >
> +			    (unsigned long)
> +			    &vma->vm_file->f_mapping->i_mmap_lock &&
> +			    (unsigned long)
> +			    &vma->vm_file->f_mapping->i_mmap_lock >
> +			    (unsigned long) i_mmap_lock_last)
> +				i_mmap_lock =
> +					&vma->vm_file->f_mapping->i_mmap_lock;
>   

So this is an O(n^2) algorithm to take the i_mmap_locks from low to high 
order?  A comment would be nice.  And O(n^2)?  Ouch.  How often is it 
called?

And is it necessary to mush lock and unlock together?  Unlock ordering 
doesn't matter, so you should just be able to have a much simpler loop, no?


> +		if (i_mmap_lock == (spinlock_t *) -1UL)
> +			break;
> +		i_mmap_lock_last = i_mmap_lock;
> +		if (lock)
> +			spin_lock(i_mmap_lock);
> +		else
> +			spin_unlock(i_mmap_lock);
> +	}
> +
> +	anon_vma_lock_last = NULL;
> +	for (;;) {
> +		spinlock_t *anon_vma_lock = (spinlock_t *) -1UL;
> +		for (vma = mm->mmap; vma; vma = vma->vm_next)
> +			if (vma->anon_vma &&
> +			    (unsigned long) anon_vma_lock >
> +			    (unsigned long) &vma->anon_vma->lock &&
> +			    (unsigned long) &vma->anon_vma->lock >
> +			    (unsigned long) anon_vma_lock_last)
> +				anon_vma_lock = &vma->anon_vma->lock;
> +		if (anon_vma_lock == (spinlock_t *) -1UL)
> +			break;
> +		anon_vma_lock_last = anon_vma_lock;
> +		if (lock)
> +			spin_lock(anon_vma_lock);
> +		else
> +			spin_unlock(anon_vma_lock);
> +	}
> +}
>   


> +
> +/*
> + * This operation locks against the VM for all pte/vma/mm related
> + * operations that could ever happen on a certain mm. This includes
> + * vmtruncate, try_to_unmap, and all page faults. The holder
> + * must not hold any mm related lock. A single task can't take more
> + * than one mm lock in a row or it would deadlock.
> + */
> +void mm_lock(struct mm_struct * mm)
> +{
> +	down_write(&mm->mmap_sem);
> +	mm_lock_unlock(mm, 1);
> +}
> +
> +void mm_unlock(struct mm_struct *mm)
> +{
> +	mm_lock_unlock(mm, 0);
> +	up_write(&mm->mmap_sem);
> +}
>
>