Re: [PATCH v12 20/31] mm: introduce vma reference counter

[Laurent Dufour]

Le 22/04/2019 Ã 22:36, Jerome Glisse a ÃcritÂ:
> On Tue, Apr 16, 2019 at 03:45:11PM +0200, Laurent Dufour wrote:
> > The final goal is to be able to use a VMA structure without holding the
> > mmap_sem and to be sure that the structure will not be freed in our back.
> >
> > The lockless use of the VMA will be done through RCU protection and thus a
> > dedicated freeing service is required to manage it asynchronously.
> >
> > As reported in a 2010's thread [1], this may impact file handling when a
> > file is still referenced while the mapping is no more there.  As the final
> > goal is to handle anonymous VMA in a speculative way and not file backed
> > mapping, we could close and free the file pointer in a synchronous way, as
> > soon as we are guaranteed to not use it without holding the mmap_sem. For
> > sanity reason, in a minimal effort, the vm_file file pointer is unset once
> > the file pointer is put.
> >
> > [1] https://lore.kernel.org/linux-mm/20100104182429.833180340@xxxxxxxxx/
> >
> > Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
>
> Using kref would have been better from my POV even with RCU freeing
> but anyway:
>
> Reviewed-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>

Thanks JÃrÃme,

I think kref is a good option here, I'll give it a try.


> > ---
> >   include/linux/mm.h       |  4 ++++
> >   include/linux/mm_types.h |  3 +++
> >   mm/internal.h            | 27 +++++++++++++++++++++++++++
> >   mm/mmap.c                | 13 +++++++++----
> >   4 files changed, 43 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index f14b2c9ddfd4..f761a9c65c74 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -529,6 +529,9 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
> >   	vma->vm_mm = mm;
> >   	vma->vm_ops = &dummy_vm_ops;
> >   	INIT_LIST_HEAD(&vma->anon_vma_chain);
> > +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> > +	atomic_set(&vma->vm_ref_count, 1);
> > +#endif
> >   }
> >   
> >   static inline void vma_set_anonymous(struct vm_area_struct *vma)
> > @@ -1418,6 +1421,7 @@ static inline void INIT_VMA(struct vm_area_struct *vma)
> >   	INIT_LIST_HEAD(&vma->anon_vma_chain);
> >   #ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> >   	seqcount_init(&vma->vm_sequence);
> > +	atomic_set(&vma->vm_ref_count, 1);
> >   #endif
> >   }
> >   
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 24b3f8ce9e42..6a6159e11a3f 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -285,6 +285,9 @@ struct vm_area_struct {
> >   	/* linked list of VM areas per task, sorted by address */
> >   	struct vm_area_struct *vm_next, *vm_prev;
> >   
> > +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> > +	atomic_t vm_ref_count;
> > +#endif
> >   	struct rb_node vm_rb;
> >   
> >   	/*
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 9eeaf2b95166..302382bed406 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -40,6 +40,33 @@ void page_writeback_init(void);
> >   
> >   vm_fault_t do_swap_page(struct vm_fault *vmf);
> >   
> > +
> > +extern void __free_vma(struct vm_area_struct *vma);
> > +
> > +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> > +static inline void get_vma(struct vm_area_struct *vma)
> > +{
> > +	atomic_inc(&vma->vm_ref_count);
> > +}
> > +
> > +static inline void put_vma(struct vm_area_struct *vma)
> > +{
> > +	if (atomic_dec_and_test(&vma->vm_ref_count))
> > +		__free_vma(vma);
> > +}
> > +
> > +#else
> > +
> > +static inline void get_vma(struct vm_area_struct *vma)
> > +{
> > +}
> > +
> > +static inline void put_vma(struct vm_area_struct *vma)
> > +{
> > +	__free_vma(vma);
> > +}
> > +#endif /* CONFIG_SPECULATIVE_PAGE_FAULT */
> > +
> >   void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
> >   		unsigned long floor, unsigned long ceiling);
> >   
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index f7f6027a7dff..c106440dcae7 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -188,6 +188,12 @@ static inline void mm_write_sequnlock(struct mm_struct *mm)
> >   }
> >   #endif /* CONFIG_SPECULATIVE_PAGE_FAULT */
> >   
> > +void __free_vma(struct vm_area_struct *vma)
> > +{
> > +	mpol_put(vma_policy(vma));
> > +	vm_area_free(vma);
> > +}
> > +
> >   /*
> >    * Close a vm structure and free it, returning the next.
> >    */
> > @@ -200,8 +206,8 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
> >   		vma->vm_ops->close(vma);
> >   	if (vma->vm_file)
> >   		fput(vma->vm_file);
> > -	mpol_put(vma_policy(vma));
> > -	vm_area_free(vma);
> > +	vma->vm_file = NULL;
> > +	put_vma(vma);
> >   	return next;
> >   }
> >   
> > @@ -990,8 +996,7 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
> >   		if (next->anon_vma)
> >   			anon_vma_merge(vma, next);
> >   		mm->map_count--;
> > -		mpol_put(vma_policy(next));
> > -		vm_area_free(next);
> > +		put_vma(next);
> >   		/*
> >   		 * In mprotect's case 6 (see comments on vma_merge),
> >   		 * we must remove another next too. It would clutter
> > --
> > 2.21.0