linux-kernel - Re: [PATCH RESEND v3 08/10] mm/vma: improve and document __is_vma_write

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Message-ID: <b267b699-f312-4387-8dc0-a14efe72561a@suse.cz>
Date: Fri, 23 Jan 2026 17:21:26 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...nel.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Mike Rapoport
 <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>,
 Shakeel Butt <shakeel.butt@...ux.dev>, Jann Horn <jannh@...gle.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev, Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>, Waiman Long <longman@...hat.com>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RESEND v3 08/10] mm/vma: improve and document
 __is_vma_write_locked()

On 1/22/26 22:55, Suren Baghdasaryan wrote:
> On Thu, Jan 22, 2026 at 5:02 AM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.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@...cle.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