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moving on to 3/10, responses below. On Thu, Jan 22, 2026 at 09:28:05AM -0800, Suren Baghdasaryan wrote: > On Thu, Jan 22, 2026 at 8:48 AM Vlastimil Babka wrote: > > > > On 1/22/26 14:01, Lorenzo Stoakes wrote: > > > 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. > > > > > > No functional change intended. > > > > > > Signed-off-by: Lorenzo Stoakes > > > > Thanks, very useful. Forgive my nitpicks :) It's because it's tricky so best > > try to be as precise as possible, I believe. > > Another thanks from me. Ack > > > > > > --- > > > include/linux/mm_types.h | 39 +++++++++++++++++++++++++++++++++++++-- > > > include/linux/mmap_lock.h | 7 +++++++ > > > mm/mmap_lock.c | 6 ++++++ > > > 3 files changed, 50 insertions(+), 2 deletions(-) > > > > > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > > > index 94de392ed3c5..e5ee66f84d9a 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 +990,41 @@ 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 the number of references taken by VMA read or > > > + * write locks. May have the VM_REFCNT_EXCLUDE_READERS_FLAG set > > > > I wonder about the "or write locks" part. The process of acquiring it uses > > VM_REFCNT_EXCLUDE_READERS_FLAG but then the writer doesn't hold a 1 > > refcount? (the sentence could be read it way IMHO) It's vma being attached > > that does, AFAIK? > > Yes, since there can be only one write-locker it only has to set > VM_REFCNT_EXCLUDE_READERS_FLAG bit to announce its presence, without > incrementing the refcount. See reply to Vlastimil, I've reworked this to: * 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. > > > > > > + * indicating that a thread has entered __vma_enter_locked() and is > > > + * waiting on any outstanding read locks to exit. > > > + * > > > + * This value can be equal to: > > > + * > > > + * 0 - Detached. > > > > Is it worth saying that readers can't increment the refcount? > > Yes, you mention that for VM_REFCNT_EXCLUDE_READERS_FLAG value. The > same IMPORTANT notice applies here. Yup already done as per reply to Vlastimil. > > > > > > + * 1 - Unlocked or write-locked. > > > > "Attached and either unlocked or write-locked." ? > > Agree. That's more specific. > Should we also mention here that unlocked vs write-locked distinction > is determined using the vm_lock_seq member? Good idea. Have updated to: * 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. Note I felt it'd be distracting to say 'vma->vm_mm->mm_lock_seq.sequence' :) mm->mm_lock_seq and hey go look at the function to see the gory details. > > > > > (see how "write-locked" isn't reflected, I argued above) > > > > > + * > > > + * >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. > > > > Ack. > > > > > + * 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 - Either an attached VMA pending > > > + * __vma_exit_locked() completion which will decrement the reference > > > + * count to one, OR a detached VMA waiting on a single spurious reader > > > + * to decrement reference count. IMPORTANT - as above, no further > > > + * readers can increment the reference count. > > > + * > > > + * > VM_REFCNT_EXCLUDE_READERS_FLAG + 1 - VMA is waiting on readers, > > > > "VMA is waiting" sounds weird? a thread might be, but VMA itself? > > (similarly in the previous paragraph) > > Maybe "VMA in the process of being write-locked or detached, which got > blocked due to the spurious readers that temporarily raised the > refcount"? > Well no, because you might have legit read locks held that you're waiting on right? That are not spurious? Can be a mix of spurious and legit (though unlikely, since by then you'd hope the READ_ONCE() check would succeed, but hey there's weird out-of-order arches out there). As per reply to Vlastimil I updated it to: * 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. Thanks, Lorenzo