Messages in this thread | | | Subject | Re: [PATCH v12 21/31] mm: Introduce find_vma_rcu() | From | Laurent Dufour <> | Date | Wed, 24 Apr 2019 16:39:01 +0200 |
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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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