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SubjectRe: [PATCH v12 21/31] mm: Introduce find_vma_rcu()
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Le 22/04/2019 à 22:57, Jerome Glisse a écrit :
> On Tue, Apr 16, 2019 at 03:45:12PM +0200, Laurent Dufour wrote:
>> This allows to search for a VMA structure without holding the mmap_sem.
>>
>> The search is repeated while the mm seqlock is changing and until we found
>> a valid VMA.
>>
>> While under the RCU protection, a reference is taken on the VMA, so the
>> caller must call put_vma() once it not more need the VMA structure.
>>
>> At the time a VMA is inserted in the MM RB tree, in vma_rb_insert(), a
>> reference is taken to the VMA by calling get_vma().
>>
>> When removing a VMA from the MM RB tree, the VMA is not release immediately
>> but at the end of the RCU grace period through vm_rcu_put(). This ensures
>> that the VMA remains allocated until the end the RCU grace period.
>>
>> Since the vm_file pointer, if valid, is released in put_vma(), there is no
>> guarantee that the file pointer will be valid on the returned VMA.
>>
>> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
>
> Minor comments about comment (i love recursion :)) see below.
>
> Reviewed-by: Jérôme Glisse <jglisse@redhat.com>

Thanks Jérôme, see my comments to your comments on my comments below ;)

>> ---
>> include/linux/mm_types.h | 1 +
>> mm/internal.h | 5 ++-
>> mm/mmap.c | 76 ++++++++++++++++++++++++++++++++++++++--
>> 3 files changed, 78 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 6a6159e11a3f..9af6694cb95d 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -287,6 +287,7 @@ struct vm_area_struct {
>>
>> #ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>> atomic_t vm_ref_count;
>> + struct rcu_head vm_rcu;
>> #endif
>> struct rb_node vm_rb;
>>
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 302382bed406..1e368e4afe3c 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -55,7 +55,10 @@ static inline void put_vma(struct vm_area_struct *vma)
>> __free_vma(vma);
>> }
>>
>> -#else
>> +extern struct vm_area_struct *find_vma_rcu(struct mm_struct *mm,
>> + unsigned long addr);
>> +
>> +#else /* CONFIG_SPECULATIVE_PAGE_FAULT */
>>
>> static inline void get_vma(struct vm_area_struct *vma)
>> {
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index c106440dcae7..34bf261dc2c8 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -179,6 +179,18 @@ static inline void mm_write_sequnlock(struct mm_struct *mm)
>> {
>> write_sequnlock(&mm->mm_seq);
>> }
>> +
>> +static void __vm_rcu_put(struct rcu_head *head)
>> +{
>> + struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
>> + vm_rcu);
>> + put_vma(vma);
>> +}
>> +static void vm_rcu_put(struct vm_area_struct *vma)
>> +{
>> + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma);
>> + call_rcu(&vma->vm_rcu, __vm_rcu_put);
>> +}
>> #else
>> static inline void mm_write_seqlock(struct mm_struct *mm)
>> {
>> @@ -190,6 +202,8 @@ static inline void mm_write_sequnlock(struct mm_struct *mm)
>>
>> void __free_vma(struct vm_area_struct *vma)
>> {
>> + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT))
>> + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma);
>> mpol_put(vma_policy(vma));
>> vm_area_free(vma);
>> }
>> @@ -197,11 +211,24 @@ void __free_vma(struct vm_area_struct *vma)
>> /*
>> * Close a vm structure and free it, returning the next.
>> */
>> -static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
>> +static struct vm_area_struct *__remove_vma(struct vm_area_struct *vma)
>> {
>> struct vm_area_struct *next = vma->vm_next;
>>
>> might_sleep();
>> + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT) &&
>> + !RB_EMPTY_NODE(&vma->vm_rb)) {
>> + /*
>> + * If the VMA is still linked in the RB tree, we must release
>> + * that reference by calling put_vma().
>> + * This should only happen when called from exit_mmap().
>> + * We forcely clear the node to satisfy the chec in
> ^
> Typo: chec -> check

Yep

>
>> + * __free_vma(). This is safe since the RB tree is not walked
>> + * anymore.
>> + */
>> + RB_CLEAR_NODE(&vma->vm_rb);
>> + put_vma(vma);
>> + }
>> if (vma->vm_ops && vma->vm_ops->close)
>> vma->vm_ops->close(vma);
>> if (vma->vm_file)
>> @@ -211,6 +238,13 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
>> return next;
>> }
>>
>> +static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
>> +{
>> + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT))
>> + VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma);
>
> Adding a comment here explaining the BUG_ON so people can understand
> what is wrong if that happens. For instance:
>
> /*
> * remove_vma() should be call only once a vma have been remove from the rbtree
> * at which point the vma->vm_rb is an empty node. The exception is when vmas
> * are destroy through exit_mmap() in which case we do not bother updating the
> * rbtree (see comment in __remove_vma()).
> */

I agree !


>> + return __remove_vma(vma);
>> +}
>> +
>> static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags,
>> struct list_head *uf);
>> SYSCALL_DEFINE1(brk, unsigned long, brk)
>> @@ -475,7 +509,7 @@ static inline void vma_rb_insert(struct vm_area_struct *vma,
>>
>> /* All rb_subtree_gap values must be consistent prior to insertion */
>> validate_mm_rb(root, NULL);
>> -
>> + get_vma(vma);
>> rb_insert_augmented(&vma->vm_rb, root, &vma_gap_callbacks);
>> }
>>
>> @@ -491,6 +525,14 @@ static void __vma_rb_erase(struct vm_area_struct *vma, struct mm_struct *mm)
>> mm_write_seqlock(mm);
>> rb_erase_augmented(&vma->vm_rb, root, &vma_gap_callbacks);
>> mm_write_sequnlock(mm); /* wmb */
>> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>> + /*
>> + * Ensure the removal is complete before clearing the node.
>> + * Matched by vma_has_changed()/handle_speculative_fault().
>> + */
>> + RB_CLEAR_NODE(&vma->vm_rb);
>> + vm_rcu_put(vma);
>> +#endif
>> }
>>
>> static __always_inline void vma_rb_erase_ignore(struct vm_area_struct *vma,
>> @@ -2331,6 +2373,34 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
>>
>> EXPORT_SYMBOL(find_vma);
>>
>> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>> +/*
>> + * Like find_vma() but under the protection of RCU and the mm sequence counter.
>> + * The vma returned has to be relaesed by the caller through the call to
>> + * put_vma()
>> + */
>> +struct vm_area_struct *find_vma_rcu(struct mm_struct *mm, unsigned long addr)
>> +{
>> + struct vm_area_struct *vma = NULL;
>> + unsigned int seq;
>> +
>> + do {
>> + if (vma)
>> + put_vma(vma);
>> +
>> + seq = read_seqbegin(&mm->mm_seq);
>> +
>> + rcu_read_lock();
>> + vma = find_vma(mm, addr);
>> + if (vma)
>> + get_vma(vma);
>> + rcu_read_unlock();
>> + } while (read_seqretry(&mm->mm_seq, seq));
>> +
>> + return vma;
>> +}
>> +#endif
>> +
>> /*
>> * Same as find_vma, but also return a pointer to the previous VMA in *pprev.
>> */
>> @@ -3231,7 +3301,7 @@ void exit_mmap(struct mm_struct *mm)
>> while (vma) {
>> if (vma->vm_flags & VM_ACCOUNT)
>> nr_accounted += vma_pages(vma);
>> - vma = remove_vma(vma);
>> + vma = __remove_vma(vma);
>> }
>> vm_unacct_memory(nr_accounted);
>> }
>> --
>> 2.21.0
>>
>

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