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SubjectRe: [PATCH v12 20/31] mm: introduce vma reference counter
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Le 22/04/2019 à 22:36, Jerome Glisse a écrit :
> 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>

Thanks Jérôme,

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
>>
>

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