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Howlett" To: Lorenzo Stoakes Cc: Andrew Morton , David Hildenbrand , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shakeel Butt , Jann Horn , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng , Waiman Long , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt Subject: Re: [PATCH v4 02/10] mm/vma: document possible vma->vm_refcnt values and reference comment Message-ID: Mail-Followup-To: "Liam R. 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 5MBk/LP4 Ozdk6Odsi8xzKrxcIa3JlZHKKQOpwkaEYp+gxRHFZ4Z1YYPrKHDDX6VH8ogvL3cF9nPd+DMqJNhpSyPuohpPcofzE9MAgvsegt4DPPOIFCvtdf+ccQqYwxcS0/CP+klTUV2wxEUubMR9mcoefTGC9Kf4yyM4RbJgXhDJo X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: * Lorenzo Stoakes [260123 15:12]: > The possible vma->vm_refcnt values are confusing and vague, explain in > detail what these can be in a comment describing the vma->vm_refcnt field > and reference this comment in various places that read/write this field. You may want to indicate what attached/detached means? Then again, they get added/removed from a few trees so it gets a bit difficult to spell out. Maybe a pointer to your nice doc file? This is certainly worth doing as is (minus that typo), so Reviewed-by: Liam R. Howlett > > No functional change intended. > > Signed-off-by: Lorenzo Stoakes > --- > include/linux/mm_types.h | 42 +++++++++++++++++++++++++++++++++++++-- > include/linux/mmap_lock.h | 7 +++++++ > mm/mmap_lock.c | 6 ++++++ > 3 files changed, 53 insertions(+), 2 deletions(-) > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index bdbf17c4f26b..12281a1128c9 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -758,7 +758,8 @@ static inline struct anon_vma_name *anon_vma_name_alloc(const char *name) > * set the VM_REFCNT_EXCLUDE_READERS_FLAG in vma->vm_refcnt to indiciate to > * vma_start_read() that the reference count should be left alone. > * > - * Once the operation is complete, this value is subtracted from vma->vm_refcnt. > + * See the comment describing vm_refcnt in vm_area_struct for details as to > + * which values the VMA reference count can be. > */ > #define VM_REFCNT_EXCLUDE_READERS_BIT (30) > #define VM_REFCNT_EXCLUDE_READERS_FLAG (1U << VM_REFCNT_EXCLUDE_READERS_BIT) > @@ -989,7 +990,44 @@ struct vm_area_struct { > struct vma_numab_state *numab_state; /* NUMA Balancing state */ > #endif > #ifdef CONFIG_PER_VMA_LOCK > - /* Unstable RCU readers are allowed to read this. */ > + /* > + * Used to keep track of firstly, whether the VMA is attached, secondly, > + * if attached, how many read locks are taken, and thirdly, if the > + * VM_REFCNT_EXCLUDE_READERS_FLAG is set, whether any read locks held > + * are currently in the process of being excluded. > + * > + * This value can be equal to: > + * > + * 0 - Detached. IMPORTANT: when the refcnt is zero, readers cannot > + * increment it. > + * > + * 1 - Attached and either unlocked or write-locked. Write locks are > + * identified via __is_vma_write_locked() which checks for equality of > + * vma->vm_lock_seq and mm->mm_lock_seq. > + * > + * >1, < VM_REFCNT_EXCLUDE_READERS_FLAG - Read-locked or (unlikely) > + * write-locked with other threads having temporarily incremented the > + * reference count prior to determining it is write-locked and > + * decrementing it again. > + * > + * VM_REFCNT_EXCLUDE_READERS_FLAG - Detached, pending > + * __vma_exit_locked() completion which will decrement the reference > + * count to zero. IMPORTANT - at this stage no further readers can > + * increment the reference count. It can only be reduced. > + * > + * VM_REFCNT_EXCLUDE_READERS_FLAG + 1 - A thread is either write-locking > + * an attached VMA and has yet to invoke __vma_exit_locked(), OR a > + * thread is detaching a VMA and is waiting on a single spurious reader > + * in order to decrement the reference count. IMPORTANT - as above, no > + * further readers can increment the reference count. > + * > + * > VM_REFCNT_EXCLUDE_READERS_FLAG + 1 - A thread is either > + * write-locking or detaching a VMA is waiting on readers to > + * exit. IMPORTANT - as above, no ruther readers can increment the > + * reference count. > + * > + * NOTE: Unstable RCU readers are allowed to read this. > + */ > refcount_t vm_refcnt ____cacheline_aligned_in_smp; > #ifdef CONFIG_DEBUG_LOCK_ALLOC > struct lockdep_map vmlock_dep_map; > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h > index 5acbd4ba1b52..a764439d0276 100644 > --- a/include/linux/mmap_lock.h > +++ b/include/linux/mmap_lock.h > @@ -130,6 +130,9 @@ static inline bool is_vma_writer_only(int refcnt) > * attached. Waiting on a detached vma happens only in > * vma_mark_detached() and is a rare case, therefore most of the time > * there will be no unnecessary wakeup. > + * > + * See the comment describing the vm_area_struct->vm_refcnt field for > + * details of possible refcnt values. > */ > return (refcnt & VM_REFCNT_EXCLUDE_READERS_FLAG) && > refcnt <= VM_REFCNT_EXCLUDE_READERS_FLAG + 1; > @@ -249,6 +252,10 @@ static inline void vma_assert_locked(struct vm_area_struct *vma) > { > unsigned int mm_lock_seq; > > + /* > + * See the comment describing the vm_area_struct->vm_refcnt field for > + * details of possible refcnt values. > + */ > VM_BUG_ON_VMA(refcount_read(&vma->vm_refcnt) <= 1 && > !__is_vma_write_locked(vma, &mm_lock_seq), vma); > } > diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c > index 1d23b48552e9..75dc098aea14 100644 > --- a/mm/mmap_lock.c > +++ b/mm/mmap_lock.c > @@ -65,6 +65,9 @@ static inline int __vma_enter_locked(struct vm_area_struct *vma, > /* > * If vma is detached then only vma_mark_attached() can raise the > * vm_refcnt. mmap_write_lock prevents racing with vma_mark_attached(). > + * > + * See the comment describing the vm_area_struct->vm_refcnt field for > + * details of possible refcnt values. > */ > if (!refcount_add_not_zero(VM_REFCNT_EXCLUDE_READERS_FLAG, &vma->vm_refcnt)) > return 0; > @@ -137,6 +140,9 @@ void vma_mark_detached(struct vm_area_struct *vma) > * before they check vm_lock_seq, realize the vma is locked and drop > * back the vm_refcnt. That is a narrow window for observing a raised > * vm_refcnt. > + * > + * See the comment describing the vm_area_struct->vm_refcnt field for > + * details of possible refcnt values. > */ > if (unlikely(!refcount_dec_and_test(&vma->vm_refcnt))) { > /* Wait until vma is detached with no readers. */ > -- > 2.52.0 >