Re: [PATCH v4] mm: use per_vma lock for MADV_DONTNEED

[David Hildenbrand <david@redhat.com>]

On 18.06.25 11:52, Barry Song wrote:
> On Wed, Jun 18, 2025 at 10:25 AM Lance Yang <lance.yang@linux.dev> wrote:
>>
>> Hi all,
>>
>> Crazy, the per-VMA lock for madvise is an absolute game-changer ;)
>>
>> On 2025/6/17 21:38, Lorenzo Stoakes wrote:
>> [...]
>>>
>>> On Sun, Jun 08, 2025 at 10:01:50AM +1200, Barry Song wrote:
>>>> From: Barry Song <v-songbaohua@oppo.com>
>>>>
>>>> Certain madvise operations, especially MADV_DONTNEED, occur far more
>>>> frequently than other madvise options, particularly in native and Java
>>>> heaps for dynamic memory management.
>>>>
>>>> Currently, the mmap_lock is always held during these operations, even when
>>>> unnecessary. This causes lock contention and can lead to severe priority
>>>> inversion, where low-priority threads—such as Android's HeapTaskDaemon—
>>>> hold the lock and block higher-priority threads.
>>>>
>>>> This patch enables the use of per-VMA locks when the advised range lies
>>>> entirely within a single VMA, avoiding the need for full VMA traversal. In
>>>> practice, userspace heaps rarely issue MADV_DONTNEED across multiple VMAs.
>>>>
>>>> Tangquan’s testing shows that over 99.5% of memory reclaimed by Android
>>>> benefits from this per-VMA lock optimization. After extended runtime,
>>>> 217,735 madvise calls from HeapTaskDaemon used the per-VMA path, while
>>>> only 1,231 fell back to mmap_lock.
>>>>
>>>> To simplify handling, the implementation falls back to the standard
>>>> mmap_lock if userfaultfd is enabled on the VMA, avoiding the complexity of
>>>> userfaultfd_remove().
>>>>
>>>> Many thanks to Lorenzo's work[1] on:
>>>> "Refactor the madvise() code to retain state about the locking mode
>>>> utilised for traversing VMAs.
>>>>
>>>> Then use this mechanism to permit VMA locking to be done later in the
>>>> madvise() logic and also to allow altering of the locking mode to permit
>>>> falling back to an mmap read lock if required."
>>>>
>>>> One important point, as pointed out by Jann[2], is that
>>>> untagged_addr_remote() requires holding mmap_lock. This is because
>>>> address tagging on x86 and RISC-V is quite complex.
>>>>
>>>> Until untagged_addr_remote() becomes atomic—which seems unlikely in
>>>> the near future—we cannot support per-VMA locks for remote processes.
>>>> So for now, only local processes are supported.
>>
>> Just to put some numbers on it, I ran a micro-benchmark with 100
>> parallel threads, where each thread calls madvise() on its own 1GiB
>> chunk of 64KiB mTHP-backed memory. The performance gain is huge:
>>
>> 1) MADV_DONTNEED saw its average time drop from 0.0508s to 0.0270s (~47%
>> faster)
>> 2) MADV_FREE     saw its average time drop from 0.3078s to 0.1095s (~64%
>> faster)
> 
> Thanks for the report, Lance. I assume your micro-benchmark includes some
> explicit or implicit operations that may require mmap_write_lock().
> As  mmap_read_lock() only waits for writers and does not block other
> mmap_read_lock() calls.

The number rather indicate that one test was run with (m)THPs enabled 
and the other not? Just a thought. The locking overhead from my 
experience is not that significant.

-- 
Cheers,

David / dhildenb