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    SubjectRe: [PATCH v12 20/31] mm: introduce vma reference counter
    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@chello.nl/
    >
    > Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>

    Using kref would have been better from my POV even with RCU freeing
    but anyway:

    Reviewed-by: Jérôme Glisse <jglisse@redhat.com>

    > ---
    > 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
    >

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