Re: [PATCH RESEND v3 08/10] mm/vma: improve and document __is_vma_write_locked()

Vlastimil Babka
2026-01-23T16:21:28Z

On 1/22/26 22:55, Suren Baghdasaryan wrote:
> On Thu, Jan 22, 2026 at 5:02 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
>>
>> The function is a little confusing, clean it up a little then add a
>> descriptive comment.
>
> I appreciate the descriptive comment but what exactly was confusing in
> this function?
>
>>
>> No functional change intended.
>>
>> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>> ---
>> include/linux/mmap_lock.h | 23 ++++++++++++++++++-----
>> 1 file changed, 18 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
>> index 873bc5f3c97c..b00d34b5ad10 100644
>> --- a/include/linux/mmap_lock.h
>> +++ b/include/linux/mmap_lock.h
>> @@ -252,17 +252,30 @@ static inline void vma_end_read(struct vm_area_struct *vma)
>> vma_refcount_put(vma);
>> }
>>
>> -/* WARNING! Can only be used if mmap_lock is expected to be write-locked */
>> -static inline bool __is_vma_write_locked(struct vm_area_struct *vma, unsigned int *mm_lock_seq)
>> +/*
>> + * Determine whether a VMA is write-locked. Must be invoked ONLY if the mmap
>> + * write lock is held.
>> + *
>> + * Returns true if write-locked, otherwise false.
>> + *
>> + * Note that mm_lock_seq is updated only if the VMA is NOT write-locked.

Could it also say to what it's updated to? Or is it too obvious?

>
> True, this does not result in a functional change because we do not
> use mm_lock_seq if __is_vma_write_locked() succeeds. However this
> seems to add additional gotcha that you need to remember. Any reason
> why?

Actually I wonder if it's really worth returning the mm_lock_seq and passing
it to __vma_start_write(), which could just determine it on its own. It
would simplify things.

>> + */
>> +static inline bool __is_vma_write_locked(struct vm_area_struct *vma,
>> + unsigned int *mm_lock_seq)
>> {
>> - mmap_assert_write_locked(vma->vm_mm);
>> + struct mm_struct *mm = vma->vm_mm;
>> + const unsigned int seq = mm->mm_lock_seq.sequence;
>> +
>> + mmap_assert_write_locked(mm);
>>
>> /*
>> * current task is holding mmap_write_lock, both vma->vm_lock_seq and
>> * mm->mm_lock_seq can't be concurrently modified.
>> */
>> - *mm_lock_seq = vma->vm_mm->mm_lock_seq.sequence;
>> - return (vma->vm_lock_seq == *mm_lock_seq);
>> + if (vma->vm_lock_seq == seq)
>> + return true;
>> + *mm_lock_seq = seq;
>> + return false;
>> }
>>
>> int __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq,
>> --
>> 2.52.0