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Howlett" , Lorenzo Stoakes , David Hildenbrand , Vlastimil Babka , Jann Horn , Suren Baghdasaryan , Lokesh Gidra , Tangquan Zheng Subject: [PATCH RFC] mm: use per_vma lock for MADV_DONTNEED Date: Tue, 27 May 2025 16:41:45 +1200 Message-Id: <20250527044145.13153-1-21cnbao@gmail.com> X-Mailer: git-send-email 2.39.3 (Apple Git-146) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable From: Barry Song 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=E2=80=94such as Android's HeapTaskDae= mon=E2=80=94 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=E2=80=99s testing shows that over 99.5% of memory reclaimed by And= roid 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(). Cc: "Liam R. Howlett" Cc: Lorenzo Stoakes Cc: David Hildenbrand Cc: Vlastimil Babka Cc: Jann Horn Cc: Suren Baghdasaryan Cc: Lokesh Gidra Cc: Tangquan Zheng Signed-off-by: Barry Song --- mm/madvise.c | 34 ++++++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+) diff --git a/mm/madvise.c b/mm/madvise.c index 8433ac9b27e0..da016a1d0434 100644 --- a/mm/madvise.c +++ b/mm/madvise.c @@ -1817,6 +1817,39 @@ int do_madvise(struct mm_struct *mm, unsigned long s= tart, size_t len_in, int beh =20 if (madvise_should_skip(start, len_in, behavior, &error)) return error; + + /* + * MADV_DONTNEED is commonly used with userspace heaps and most often + * affects a single VMA. In these cases, we can use per-VMA locks to + * reduce contention on the mmap_lock. + */ + if (behavior =3D=3D MADV_DONTNEED || behavior =3D=3D MADV_DONTNEED_LOCKED= ) { + struct vm_area_struct *prev, *vma; + unsigned long untagged_start, end; + + untagged_start =3D untagged_addr(start); + end =3D untagged_start + len_in; + vma =3D lock_vma_under_rcu(mm, untagged_start); + if (!vma) + goto lock; + if (end > vma->vm_end || userfaultfd_armed(vma)) { + vma_end_read(vma); + goto lock; + } + if (unlikely(!can_modify_vma_madv(vma, behavior))) { + error =3D -EPERM; + vma_end_read(vma); + goto out; + } + madvise_init_tlb(&madv_behavior, mm); + error =3D madvise_dontneed_free(vma, &prev, untagged_start, + end, &madv_behavior); + madvise_finish_tlb(&madv_behavior); + vma_end_read(vma); + goto out; + } + +lock: error =3D madvise_lock(mm, behavior); if (error) return error; @@ -1825,6 +1858,7 @@ int do_madvise(struct mm_struct *mm, unsigned long st= art, size_t len_in, int beh madvise_finish_tlb(&madv_behavior); madvise_unlock(mm, behavior); =20 +out: return error; } =20 --=20 2.39.3 (Apple Git-146)