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

[Qi Zheng]

On 6/8/25 6:01 AM, Barry Song wrote:
> From: Barry Song <v-songbaohua@xxxxxxxx>
>
> 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.
>
> Link: https://lore.kernel.org/all/0b96ce61-a52c-4036-b5b6-5c50783db51f@lucifer.local/ [1]
> Link: https://lore.kernel.org/all/CAG48ez11zi-1jicHUZtLhyoNPGGVB+ROeAJCUw48bsjk4bbEkA@xxxxxxxxxxxxxx/ [2]
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Jann Horn <jannh@xxxxxxxxxx>
> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Cc: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
> Cc: Tangquan Zheng <zhengtangquan@xxxxxxxx>
> Cc: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> ---
>   -v4:
>   * collect Lorenzo's RB;
>   * use visit() for per-vma path
>
>   mm/madvise.c | 195 ++++++++++++++++++++++++++++++++++++++-------------
>   1 file changed, 147 insertions(+), 48 deletions(-)

Acked-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>

Thanks.