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@xxxxxxxxxxxxx>Minor comments about comment (i love recursion :)) see below. Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
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_FAULTatomic_t vm_ref_count; + struct rcu_head vm_rcu; #endif struct rb_node vm_rb;diff --git a/mm/internal.h b/mm/internal.hindex 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
